Identification and validation of N-acetyltransferase 2 as an insulin sensitivity gene

Decreased insulin sensitivity, also referred to as insulin resistance (IR), is a fundamental abnormality in patients with type 2 diabetes and a risk factor for cardiovascular disease. While IR predisposition is heritable, the genetic basis remains largely unknown. The GENEticS of Insulin Sensitivity consortium conducted a genome-wide association study (GWAS) for direct measures of insulin sensitivity, such as euglycemic clamp or insulin suppression test, in 2,764 European individuals, with replication in an additional 2,860 individuals. The presence of a nonsynonymous variant of N-acetyltransferase 2 (NAT2) [rs1208 (803A>G, K268R)] was strongly associated with decreased insulin sensitivity that was independent of BMI. The rs1208 "A" allele was nominally associated with IR-related traits, including increased fasting glucose, hemoglobin A1C, total and LDL cholesterol, triglycerides, and coronary artery disease. NAT2 acetylates arylamine and hydrazine drugs and carcinogens, but predicted acetylator NAT2 phenotypes were not associated with insulin sensitivity. In a murine adipocyte cell line, silencing of NAT2 ortholog Nat1 decreased insulin-mediated glucose uptake, increased basal and isoproterenol-stimulated lipolysis, and decreased adipocyte differentiation, while Nat1 overexpression produced opposite effects. Nat1-deficient mice had elevations in fasting blood glucose, insulin, and triglycerides and decreased insulin sensitivity, as measured by glucose and insulin tolerance tests, with intermediate effects in Nat1 heterozygote mice. Our results support a role for NAT2 in insulin sensitivity.

5-methyl-tetrahydrofolate and the S-adenosylmethionine cycle in C57BL/6J mouse tissues: gender differences and effects of arylamine N-acetyltransferase-1 deletion

Folate catabolism involves cleavage of the C⁹-N¹⁰ bond to form p-aminobenzoylgluamate (PABG) and pterin. PABG is then acetylated by human arylamine N-acetyltransferase 1 (NAT1) before excretion in the urine. Mice null for the murine NAT1 homolog (Nat2) show several phenotypes consistent with altered folate homeostasis. However, the exact role of Nat2 in the folate pathway in vivo has not been reported. Here, we examined the effects of Nat2 deletion in male and female mice on the tissue levels of 5-methyl-tetrahydrofolate and the methionine-S-adenosylmethionine cycle. We found significant gender differences in hepatic and renal homocysteine, S-adenosylmethionine and methionine levels consistent with a more active methionine-S-adenosylmethionine cycle in female tissues. In addition, methionine levels were significantly higher in female liver and kidney. PABG was higher in female liver tissue but lower in kidney compared to male tissues. In addition, qPCR of mRNA extracted from liver tissue suggested a significantly lower level of Nat2 expression in female animals. Deletion of Nat2 affected liver 5- methyl-tetrahydrofolate in female mice but had little effect on other components of the methionine-S-adenosylmethionine cycle. No N-acetyl-PABG was observed in any tissues in Nat2 null mice, consistent with the role of Nat2 in PABG acetylation. Surprisingly, tissue PABG levels were similar between wild type and Nat2 null mice. These results show that Nat2 is not required to maintain tissue PABG homeostasis in vivo under normal conditions.

N-acetyltransferase polymorphism and risk of colorectal adenoma and cancer: a pooled analysis of variations from 59 studies

BACKGROUND

There have been an increasing number of studies with evidence suggesting that the N-acetyltransferase 1 (NAT1) and N-acetyltransferase 2 (NAT2) genotypes may be implicated in the development of colorectal cancer (CRC) and colorectal adenoma (CRA). So far the published data on this association has remained controversial, however. We performed a meta-analysis of case-cohort and case-control studies using a subset of the published data, with an aim to derive a better understanding of the underlying relationship.

METHODS/PRINCIPAL FINDINGS

A literature search was performed using Medline database for relevant studies published through October 31, 2011. A total of 39 publications were selected for this meta-analysis, including 11,724 cases and 16,215 controls for CRC, and 3,701 cases and 5,149 controls for CRA. In our pooled analysis of all these studies, the results of our meta-analysis suggested that the NAT1 genotype was not significantly associated with an elevated CRC risk (OR 0.99, 95% CI 0.91-1.07). We also found that individuals with the rapid NAT2 genotype did have an elevated risk of CRC (OR 1.07, 95% CI 1.01-1.13). There was no evidence for an association between the NAT1 and 2 rapid genotype and an elevated CRA risk (NAT1: OR 1.14, 95% CI 0.99-1.29; NAT2: OR 0.94, 95% CI 0.86-1.03).

CONCLUSION

This meta-analysis suggests that individuals with NAT2 genotype had an elevated risk of CRC. There was no evidence for the association between NAT1 and 2 rapid genotype and CRA risk.

Biosynthetic Pathways of the Endocannabinoid Anandamide

Anandamide (=N-arachidonoylethanolamine) is the first discovered endocannabinoid, and belongs to the class of bioactive, long-chain N-acylethanolamines (NAEs). In animal tissues, anandamide is principally formed together with other NAEs from glycerophospholipid by two successive enzymatic reactions: 1) N-acylation of phosphatidylethanolamine to generate N-acylphosphatidylethanolamine (NAPE) by Ca2+-dependent N-acyltransferase; 2) release of NAE from NAPE by a phosphodiesterase of the phospholipase D type (NAPE-PLD). Although these anandamide-synthesizing enzymes were poorly understood until recently, our cDNA cloning of NAPE-PLD in 2004 enabled molecular-biological approaches to the enzymes. NAPE-PLD is a member of the metallo-beta-lactamase family, which specifically hydrolyzes NAPE among glycerophospholipids, and appears to be constitutively active. Mutagenesis studies suggested that the enzyme functions through a mechanism similar to those of other members of the family. NAPE-PLD is widely expressed in animal tissues, including various regions in rat brain. Its expression level in the brain is very low at birth, and remarkably increases with development. Analysis of NAPE-PLD-deficient mice and other recent studies revealed the presence of NAPE-PLD-independent pathways for the anandamide formation. Furthermore, calcium-independent N-acyltransferase was discovered and characterized. In this article, we will review recent progress in the studies on these enzymes responsible for the biosynthesis of anandamide and other NAEs.

Contribution of the N-acetyltransferase 2 polymorphism NAT2*6A to age-related hearing impairment

BACKGROUND

Age-related hearing impairment (ARHI) is the most common sensory impairment in older people, affecting 50% of those aged 80 years. The proportion of older people is increasing in the general population, and as a consequence, the number of people affected with ARHI is growing. ARHI is a complex disorder, with both environmental and genetic factors contributing to the disease. The first studies to elucidate these genetic factors were recently performed, resulting in the identification of the first two susceptibility genes for ARHI, NAT2 and KCNQ4.

METHODS

In the present study, the association between ARHI and polymorphisms in genes that contribute to the defence against reactive oxygen species, including GSTT1, GSTM1 and NAT2, was tested. Samples originated from seven different countries and were combined into two test population samples, the general European population and the Finnish population. Two distinct phenotypes for ARHI were studied, Z(low) and Z(high), representing hearing in the low and high frequencies, respectively. Statistical analysis was performed for single polymorphisms (GSTM1, GSTT1, NAT2*5A, NAT2*6A, and NAT2*7A), haplotypes, and gene-environment and gene-gene interactions.

RESULTS

We found an association between ARHI and GSTT1 and GSTM1 in the Finnish population sample, and with NAT2*6A in the general European population sample. The latter finding replicates previously published data.

CONCLUSION

As replication is considered the ultimate proof of true associations in the study of complex disorders, this study provides further support for the involvement of NAT2*6A in ARHI.

Functional characterization of single-nucleotide polymorphisms and haplotypes of human N-acetyltransferase 2

Human N-acetyltransferase 2 (NAT2) is polymorphic in humans and may associate with cancer risk by modifying individual susceptibility to cancers from carcinogen exposure. Since molecular epidemiological studies investigating these associations usually include determining NAT2 single-nucleotide polymorphisms (SNPs), haplotypes or genotypes, their conclusions can be compromised by the uncertainty of genotype-phenotype relationships. We characterized NAT2 SNPs and haplotypes by cloning and expressing recombinant NAT2 allozymes in mammalian cells. The reference and variant recombinant NAT2 allozymes were characterized for arylamine N-acetylation and O-acetylation of N-hydroxy-arylamines. SNPs and haplotypes that conferred reduced enzymatic activity did so by reducing NAT2 protein without changing NAT2 mRNA levels. Among SNPs that reduced catalytic activity, G191A (R64Q), G590A (R197Q) and G857A (G286E) reduced protein half-life but T341C (I114T), G499A (E167K) and A411T (L137F) did not. G857A (G286E) and the major haplotype possessing this SNP (NAT2 7B) altered the affinity to both substrate and cofactor acetyl coenzyme A, resulting in reduced catalytic activity toward some substrates but not others. Our results suggest that coding region SNPs confer slow acetylator phenotype by multiple mechanisms that also may vary with arylamine exposures.

Arylamine N-acetyltransferase responsible for acetylation of 2-aminophenols in Streptomyces griseus

An arylamine N-acetyltransferase (NAT) responsible for the N acetylation of exogenous 3-amino-4-hydroxybenzoic acid in Streptomyces griseus was identified and characterized. This enzyme was distinct from other eukaryotic and bacterial NATs in that it acetylated various 2-aminophenol derivatives more effectively than it acetylated 5-aminosalicylic acid, and thus it may be involved in the metabolism of xenobiotic compounds.

The Urinary Metabolite Profile of the Dietary Carcinogen 2-Amino-1-Methyl-6-Phenylimidazo[4,5-b]Pyridine Is Predictive of Colon DNA Adducts after a Low-Dose Exposure in Humans

Epidemiologic evidence indicates that exposure to heterocyclic amines in the diet is an important risk factor for the development of colon cancer. Well-done cooked meats contain significant levels of heterocyclic amines, which have been shown to cause cancer in laboratory animals. To better understand the mechanisms of heterocyclic amine bioactivation in humans, the most mass abundant heterocyclic amine, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), was used to assess the relationship between PhIP metabolism and DNA adduct formation. Ten human volunteers where administered a dietary relevant dose of [(14)C]PhIP 48 to 72 hours before surgery to remove colon tumors. Urine was collected for 24 hours after dosing for metabolite analysis, and DNA was extracted from colon tissue and analyzed by accelerator mass spectrometry for DNA adducts. All 10 subjects were phenotyped for cytochrome P4501A2 (CYP1A2), N-acetyltransferase 2, and sulfotransferase 1A1 enzyme activity. Twelve PhIP metabolites were detected in the urine samples. The most abundant metabolite in all volunteers was N-hydroxy-PhIP-N(2)-glucuronide. Metabolite levels varied significantly between the volunteers. Interindividual differences in colon DNA adducts levels were observed between each individual. The data showed that individuals with a rapid CYP1A2 phenotype and high levels of urinary N-hydroxy-PhIP-N(2)-glucuronide had the lowest level of colon PhIP-DNA adducts. This suggests that glucuronidation plays a significant role in detoxifying N-hydroxy-PhIP. The levels of urinary N-hydroxy-PhIP-N(2)-glucuronide were negatively correlated to colon DNA adduct levels. Although it is difficult to make definite conclusions from a small data set, the results from this pilot study have encouraged further investigations using a much larger study group.

Arylamine N-acetyltransferase Aggregation and Constitutive Ubiquitylation

Arylamine N-acetyltransferases (NAT1 and NAT2) acetylate and detoxify arylamine carcinogens. Humans harboring certain genetic variations within the NAT genes exhibit increased likelihood of developing various cancer types, especially urinary bladder cancer. Such DNA polymorphisms result in protein products with reduced cellular activity, which is proposed to be due to their constitutive ubiquitylation and enhanced proteasomal degradation. To identify the properties that lead to the reduced cellular activity of certain NAT variants, we introduced one such polymorphism into the human NAT1 ortholog hamster NAT2. The polymorphism chosen was human NAT1*17, which results in the replacement of R64 with a tryptophan residue, and we demonstrate this substitution to cause hamster NAT2 to be constitutively ubiquitylated. Biophysical characterization of the hamster NAT2 R64W variant revealed that its overall protein structure and thermostability are not compromised. In addition, we used steady-state kinetics experiments to demonstrate that the R64W mutation does not interfere with NAT catalysis in vitro. Hence, the constitutive ubiquitylation of this variant is not caused by its inability to be acetylated. Instead, we demonstrate this mutation to cause the hamster NAT2 protein to aggregate in vitro and in vivo. Importantly, we tested and confirmed that the R64W mutation also causes human NAT1 to aggregate in cultured cells. By using homology modeling, we demonstrate that R64 is located at a peripheral location, which provides an explanation for how the NAT protein structure is not significantly disturbed by its mutation to tryptophan. Altogether, we provide fundamental information on why humans harboring certain NAT variants exhibit reduced acetylation capabilities.

Do cross-registry comparisons of Black and White Americans provide support for N-acetylation as an important determinant for urinary bladder and other tobacco-related cancers?

Tobacco smoking is an unequivocal risk factor for cancers of the larynx, lung, pancreas and urinary bladder. Whereas African-Americans demonstrate higher laryngeal, lung and pancreatic cancer rates than their Caucasian-American counterparts, they paradoxically have only approximately half of the urinary bladder incidences. One possible explanation is their N-acetyltransferase (NAT) status, since this enzyme is responsible for metabolism of arylamines in smoke and blacks are reported to have a higher rate for rapid acetylation than whites. However, other tobacco-related cancers are also linked to slow acetylation so that African-Americans might therefore also be expected to have lower incidences of other tobacco-related cancers. The present investigation was conducted with data from Cancer Incidence in Five Continents Vol VIII to assess whether there might be correlations between incidence rates for four major cancers across registries in the United States. Cluster analysis demonstrated clear separation of the white and black populations for all states, and significant correlations were observed between bladder and laryngeal cancers, and also for lung and laryngeal cancers, for both Blacks and Whites. Striking similarities in the plots for urinary bladder incidence against all three of the other cancers suggests the existence of a factor specific to the bladder. A review of black-white ratios for cancer incidences in all major body sites in both sexes and the published literature for NAT polymorphisms provided evidence that this might indeed be arylamine exposure, although other factors could also be involved.

Characterization of ARD1 variants in mammalian cells

Mouse ARD1 (mARD1) has been reported to negatively regulate the hypoxia-inducible factor 1alpha (HIF-1alpha) protein by acetylating a lysine residue and enhancing HIF-1alpha ubiquitination and degradation. However, it was recently reported that human ARD1 (hARD1) does not affect HIF-1alpha stability. To further explore the activities of the two orthologs, three mouse (mARD1(198), mARD1(225), mARD1(235)) and two human (hARD1(131), hARD1(235)) variants were identified and characterized. Among these, mARD1(225) was previously reported as a novel negative regulator of HIF-1alpha. Amino acid sequence analysis showed that the C-terminal region (aa 158-225) of mARD1(225) completely differs from those of mouse and human ARD1(235), although all three proteins share a well-conserved N-acetyltransferase domain (aa 45-130). The effects of ARD1 variants were evaluated with respect to HIF-1alpha stability and acetylation activity. Interestingly, mARD1(225) strongly decreased the level of HIF-1alpha and increased the extent of acetylation, whereas mARD1(235) and hARD1(235) variants had a much weaker effect on HIF-1alpha stability and acetylation. These results suggest that ARD1 variants might have different effects on HIF-1alpha stability and acetylation, which may reflect diverse biological functions that remain to be determined.

SLI1 (YGR212W) is a major gene conferring resistance to the sphingolipid biosynthesis inhibitor ISP-1, and encodes an ISP-1 N-acetyltransferase in yeast

ISP-1 (myriocin) is a potent inhibitor of serine palmitoyltransferase, the primary enzyme of sphingolipid biosynthesis, and is a useful tool for studying the biological functions of sphingolipids in both mammals and yeast (Saccharomyces cerevisiae). In a previous study, we cloned yeast multicopy suppressor genes for ISP-1, and one of these, YPK1/SLI2, was shown to encode a serine/threonine kinase which is a yeast homologue of mammalian SGK1 (serum/glucocorticoid-regulated kinase 1). In the present study, another gene, termed SLI1 (YGR212W; GenBank accession number CAA97239.1), was characterized. Sli1p has weak similarity to Atf1p and Atf2p, which are alcohol acetyltransferases. Although a sli1-null strain grew normally, the IC50 of ISP-1 for the growth of this strain was markedly decreased compared with that for the parental strain, indicating that Sli1p is a major contributor to ISP-1 resistance in yeast. On a sli1-null background, the increase in resistance to ISP-1 induced by YPK1 gene transfection was almost abolished. These data indicate that Sli1p co-operates with Ypk1p in mediating resistance to ISP-1 in yeast. Sli1p was found to convert ISP-1 into N-acetyl-ISP-1 in vitro. Furthermore, N-acetyl-ISP-1 did not share the ability of ISP-1 to inhibit the growth of yeast cells, and the serine palmitoyltransferase inhibitory activity of N-acetyl-ISP-1 was much lower than that of ISP-1. These data suggest that Sli1p inactivates ISP-1 due to its N-acetyltransferase activity towards ISP-1.

Circadian rhythm and photic control of cAMP level in chick retinal cell cultures: a mechanism for coupling the circadian oscillator to the melatonin-synthesizing enzyme, arylalkylamine N-acetyltransferase, in photoreceptor cells

Arylalkylamine N-acetyltransferase (AANAT) is the penultimate and key regulatory enzyme in the melatonin biosynthetic pathway. In chicken retina in vivo, AANAT is expressed in a circadian fashion, primarily in photoreceptor cells. AANAT activity is high at night in darkness, low during the daytime, and suppressed by light exposure at night. In the present study, we investigated the circadian and photic regulation of adenosine 3',5'-monophosphate (cAMP) in cultured retinal cells entrained to a daily light-dark (LD) cycle, as well as the role of Ca(2+) and cAMP in the regulation of AANAT activity. Similar to AANAT activity, cAMP levels fluctuate in a daily fashion, with high levels at night in darkness and low levels during the day in light. This daily fluctuation continued with reduced amplitude in constant (24 h/day) darkness (DD). These changes in cAMP appear to be causally related to control of AANAT activity. Adenylyl cyclase and protein kinase A inhibitors suppress the nocturnal increase of AANAT in DD, while 8Br-cAMP augments it. The nocturnal increase of AANAT activity also involves Ca(2+) influx, as it is inhibited by nitrendipine, an inhibitor of L-type voltage-gated channels, and augmented by Bay K 8644, a Ca(2+) channel agonist. The effect of Bay K 8644 was antagonized by the adenylyl cyclase inhibitor MDL 12330A, suggesting a link between Ca(2+) influx, cAMP formation, and AANAT activity in retinal cells. Light exposure at night, which rapidly suppresses AANAT activity, also suppressed cAMP levels. The effect of light on AANAT activity was reversed by Bay K 8644, 8Br-cAMP, and the proteasome inhibitor lactacystin. These results indicate a dynamic interplay of circadian oscillators and light in the regulation of cAMP levels and AANAT activity in photoreceptor cells.

Use of genetically manipulated Salmonella typhimurium strains to evaluate the role of sulfotransferases and acetyltransferases in nitrofen mutagenicity

Nitrofen had been used as a herbicide, until its carcinogenic and teratogenic activity in rodents was detected. A food contamination occurring in 2002 in Germany led to the initiation of new studies in order to better understand the potential risk for humans. Nitrofen is a nitroarene and as such might be activated to a mutagen via reduction to the corresponding hydroxylamine and subsequent formation of a reactive acetic or sulfuric acid ester. Therefore, we have investigated the mutagenicity of nitrofen in Salmonella typhimurium strains engineered for the expression of all human xenobiotic-metabolizing sulfotransferases (SULTs) and acetyltransferases (NATs) identified. Nitrofen was inactive in the parental strains TA1538, TA98 and TA100, but was mutagenic even at low doses when human sulfotransferase SULT1A1 (the major broad-spectrum phenol SULT) was expressed in these strains, but not when it was expressed in a TA1538-derived strain deficient in an endogenous nitroreductase. Several other human SULTs (in particular 1A3 and 1C1) as well as human NAT2 (unlike NAT1) also activated nitrofen, but were markedly less efficient than SULT1A1. Likewise, expression of rat and mouse SULT1A1 led to weaker mutagenic activity of nitrofen than expression of the corresponding human enzyme. An endogenous acetyltransferase only activated nitrofen to a mutagen when it was strongly over-expressed in the TA98-derived strain YG1024. Thus, humans might be more susceptible to the carcinogenic effects of nitrofen than mice and rats, which have been used in long-term studies. The fact that several SULTs show particular high expression in fetal tissues suggests that this activation pathway may also play a role in the teratogenic effects observed.

Composition and function of the eukaryotic N-terminal acetyltransferase subunits

Saccharomyces cerevisiae contains three N-terminal acetyltransferases (NATs), NatA, NatB, and NatC, composed of the following catalytic and auxiliary subunits: Ard1p and Nat1p (NatA); Nat3p and Mdm20p (NatB); and Mak3p, Mak10, and Mak31p (NatC). The overall patterns of N-terminally acetylated proteins and NAT orthologous genes suggest that yeast and higher eukaryotes have similar systems for N-terminal acetylation. The differential expression of certain NAT subunits during development or in carcinomas of higher eukaryotes suggests that the NATs are more highly expressed in cells undergoing rapid protein synthesis. Although Mak3p is functionally the same in yeast and plants, findings with TE2 (a human Ard1p ortholog) and Tbdn100 (a mouse Nat1p ortholog) suggest that certain of the NAT subunits may have functions other than their role in NATs or that these orthologs are not functionally equivalent. Thus, the vertebrate NATs remain to be definitively identified, and, furthermore, it remains to be seen if any of the yeast NATs contribute to other functions.

Significant association of the arylalkylamine N-acetyltransferase ( AA-NAT) gene with delayed sleep phase syndrome

Arylalkylamine N-acetyltransferase (AA-NAT) is a rate-limiting enzyme in melatonin hormone synthesis and participates in daily oscillations of the melatonin level. We studied the association between the AA-NAT gene and delayed sleep phase syndrome (DSPS). Results indicate that there is a significant difference in allele positivity at the single nucleotide polymorphism involved in an amino acid substitution from alanine to threonine at position 129 between patients with DSPS and healthy controls. The frequency of the 129 threonine allele is significantly higher in the patients than in the controls ( P=0.0029). The data suggest that AA-NAT could be a susceptibility gene for DSPS.

A pharmacogenetic study to investigate the role of dietary carcinogens in the etiology of colorectal cancer

Susceptibility to colorectal cancer, one of the most common forms of cancer in the Western world, has been associated with several environmental and dietary risk factors. Dietary exposure to food derived heterocyclic amine carcinogens and polycyclic aromatic hydrocarbons have been proposed as specific risk factors. Many polymorphic Phase I and Phase II drug metabolizing enzymes are responsible for the metabolism and disposition of these compounds and it is therefore possible that inheritance of specific allelic variants of these enzymes may influence colorectal cancer susceptibility. In a multicenter case-control study, 490 colorectal cancer patients and 593 controls (433 matched case-control pairs) were genotyped for common polymorphisms in the cytochrome P450 (CYP1A1, CYP1A2, CYP1B1, CYP2A6, CYP2C9, CYP2C19 and CYP2D6), glutathione S-transferase (GSTM1, GSTP1 and GSTT1), sulfotransferase (SULT1A1 and SULT1A2), N-acetyl transferase 2 (NAT2), NAD(P)H:quinone oxidoreductase (NQO1), methylenetetrahydrofolate reductase (MTHFR), and microsomal epoxide hydrolase (EPHX1) genes. Matched case-control analysis identified alleles associated with higher colorectal cancer risk as carriage of CYP1A1*2C (OR = 2.15, 95% CI 1.36-3.39) and homozygosity for GSTM1*2/*2 (OR = 1.53, 95% CI 1.16-2.02). In contrast, inheritance of the CYP2A6*2 (OR = 0.51, 95% CI 0.28-1.06), CYP2C19*2 (OR = 0.72, 95% CI 0.52-0.98) and the EPHX1(His113) alleles were associated with reduced cancer risk. We found no association with colorectal cancer risk with NAT2 genotype or any of the other polymorphic genes associated with the metabolism and disposition of heterocyclic amine carcinogens. This data suggests that heterocyclic amines do not play an important role in the aetiology of colorectal cancer but that exposure to other carcinogens such as polycyclic aromatic hydrocarbons may be important determinants of cancer risk.

Molecular genetics and function of NAT1 and NAT2: role in aromatic amine metabolism and carcinogenesis

Aromatic and heterocyclic amines require metabolic activation to electrophilic intermediates that initiate carcinogenesis. N-Acetyltransferase 1 (NAT1) and 2 (NAT2) are important enzymes in the biotransformation of these carcinogens and exhibit genetic polymorphism. Human NAT1 and NAT2 alleles are listed at: http://www.louisville.edu/medschool/pharmacology/NAT.html by an international gene nomenclature committee. The high frequency of the NAT1 and NAT2 acetylation polymorphisms in human populations together with ubiquitous exposure to aromatic and heterocyclic amines suggest that NAT1 and NAT2 acetylator genotypes are important modifiers of human cancer susceptibility. For cancers in which N-acetylation is a detoxification step such as aromatic amine-related urinary bladder cancer, NAT2 slow acetylator phenotype is at higher risk. Multiple studies have shown that the urinary bladder cancer risk is particularly high in the slowest NAT2 acetylator phenotype or genotype (NAT2(*)5). In contrast, for cancers in which N-acetylation is negligible and O-acetylation is an activation step such as for heterocyclic amine-related colon cancer, NAT2 rapid acetylator phenotype is at higher risk. Although studies have found associations between NAT1 genotype and various cancers, the findings are less consistent and are not well understood. Since cancer risk requires exposure to aromatic and/or heterocyclic amine carcinogens modified by NAT1 and/or NAT2 acetylator genotype, the results from human epidemiology studies are dependent upon the quality and accuracy of the exposure assessment and genotype determination. Conclusions require understanding the relationship between genotype and phenotype, as well as the role of genetic variation in carcinogen metabolism, DNA repair, and host susceptibility. Investigations have been carried out in rapid and slow acetylator rodent models in which both exposure and genetic variability are tightly controlled. Human NAT1 and NAT2 alleles have been characterized by recombinant expression to further understand the effects of nucleotide polymorphisms on function and phenotype.

GSTT1 null genotype increases risk of premenopausal breast cancer

The NAT2, GSTM1 and GSTT1 genes are known candidate cancer susceptibility markers and have been investigated in breast cancer susceptibility with conflicting results. We conducted a case-control study to investigate the role of NAT2, GSTM1 and GSTT1 in premenopausal breast cancer. Women with the GSTT1 null genotype were found to have a significant 3.15-fold increased risk of breast cancer (95% CI = 1.7-5.8), while GSTM1 and NAT2 genotypes were not associated with breast cancer risk. Our results suggest that the GSTT1 null genotype may play a role in early onset breast cancer.

Functional genomics of C190T single nucleotide polymorphism in human N-acetyltransferase 2

N-acetyltransferase 2 (NAT2) catalyzes N-acetylation and O-acetylation of many drugs and environmental carcinogens. Genetic polymorphisms in the NAT2 gene have been associated with differential susceptibility to cancers and drug toxicity from these compounds. Single nucleotide polymorphisms (SNPs) have been identified in the human NAT2 coding region. A new allele, NAT2*19, possessing the C190T (R64W) exchange, was recently identified. In order to understand the effect of this new SNP, recombinant NAT2*4 (reference) and NAT2*19 were expressed in yeast (Schizosaccharomyces pombe). The C190T (R64W) SNP in NAT2*19 caused substantial reduction in the NAT2 protein level and stability, but did not cause significant reduction in transformation efficiency or mRNA level. The enzymatic activities for N-acetylation of two arylamine carcinogens (2-aminofluorene, 4-aminobiphenyl), and a sulfonamide drug (sulfamethazine) were over 100-fold lower for NAT2 19 compared to reference NAT2 4. Kinetic studies showed a reduction in Vmax but no significant change in substrate Km. In addition, the SNP caused significant reduction in the O-acetylation of the N-hydroxy-2-amino-1-methyl-6-phenylimidazo [4,5-b] pyridine. These results show that NAT2*19 possessing the C190T (R64W) SNP encodes a slow acetylator phenotype for both N- and O-acetylation, due to a reduction in the amount and stability of the NAT2 19 allozyme.

Food access schedule and diet composition alter rhythmicity of serum melatonin and pineal NAT activity

This study investigated the effect of dietary composition and food access schedule on the rhythmicity of serum melatonin and pineal N-acetyltransferase (NAT) activity. Wistar rats maintained on a 12:12 h light-dark cycle were assigned to two dietary groups: a group fed rat chow and a group fed a choice between a protein-rich and a carbohydrate-rich diet. Each dietary group was further divided based on feeding schedule, with food available between 0800 and 1600 h or ad lib access to food. Regardless of dietary condition, total food and carbohydrate intake of rats having free access to food was higher than under the restricted food access schedule. Protein intake of rats fed the dietary choice was lower with the restricted access than in the free access. In rats fed the dietary choice, melatonin levels and NAT activity were significantly decreased with restricted access compared to free access. Such results were not found in rats offered restricted chow. This study suggests that the rhythms of melatonin secretion and NAT activity can be altered by dietary composition.

Role of a pineal cAMP-operated arylalkylamine N-acetyltransferase/14-3-3-binding switch in melatonin synthesis

The daily rhythm in melatonin levels is controlled by cAMP through actions on the penultimate enzyme in melatonin synthesis, arylalkylamine N-acetyltransferase (AANAT; serotonin N-acetyltransferase, EC ). Results presented here describe a regulatory/binding sequence in AANAT that encodes a cAMP-operated binding switch through which cAMP-regulated protein kinase-catalyzed phosphorylation [RRHTLPAN --> RRHpTLPAN] promotes formation of a complex with 14-3-3 proteins. Formation of this AANAT/14-3-3 complex enhances melatonin production by shielding AANAT from dephosphorylation and/or proteolysis and by decreasing the K(m) for 5-hydroxytryptamine (serotonin). Similar switches could play a role in cAMP signal transduction in other biological systems.

Influence of photoperiod on N-acetyltransferase activity and melatonin in the fiddler crab Uca pugilator

Melatonin and N-acetyltransferase (NAT) activity were measured in the eyestalks of fiddler crabs acclimated to various photoperiods: constant light, a L:D 12:12 h photoperiod, or constant dark. Following acclimation, eyestalks were collected every 3 h over a 24-h period; they were assayed for melatonin with a radioimmunoassay and for NAT activity with a radioenzymatic assay. In constant light, melatonin levels increased at 1300 h, from 142 to 431 pg x mg(-1) eyestalk; NAT activity increased concurrently, from 97 to 203 pmol x h(-1) x mg(-1) eyestalk, and both remained elevated until 0400 h. In the L:D 12:12 h photoperiod, melatonin levels increased at 1300 h from 28 to 230 pg x mg(-1) eyestalk, and though NAT activity increased significantly, from 80 to 122 pmol x h(-1) x mg(-1) eyestalk, an even greater increase occurred at 0400 h, when melatonin levels were low. In constant dark, melatonin levels increased at 1600 h, from 22 to 196 pg x mg(-1) eyestalk, with a concurrent increase in NAT activity from 93 to 140 pmol x mg(-1) x h(-1) eyestalk. However, the second peak in melatonin (111 pg x mg(-1)), occurring at 0400 h, was out of phase with the second peak of NAT activity (113 pmol x mg(-1) x h(-1) eyestalk) which occurred at 0700 h. NAT may be a rate-limiting step in melatonin synthesis in fiddler crabs under some conditions (constant light and the 1300 h peak in constant dark); however, NAT activity correlates poorly with melatonin levels in a L:D 12:12 h photoperiod and in constant dark relative to the 0400 h melatonin peak.

Functional characterization of human N-acetyltransferase 2 (NAT2) single nucleotide polymorphisms

N-Acetyltransferase 2 (NAT2) catalyses the activation and/or deactivation of a variety of aromatic amine drugs and carcinogens. Polymorphisms in the N-acetyltransferase 2 (NAT2) gene have been associated with a variety of drug-induced toxicities, as well as cancer in various tissues. Eleven single nucleotide polymorphisms (SNPs) have been identified in the NAT2 coding region, but the specific effects of each of these SNPs on expression of NAT2 protein and N-acetyltransferase enzymatic activity are poorly understood. To investigate the functional consequences of SNPs in the NAT2 coding region, reference NAT2*4 and NAT2 variant alleles possessing one of the 11 SNPs in the NAT2 coding region were cloned and expressed in yeast (Schizosaccharomyces pombe). Reductions in catalytic activity for the N-acetylation of a sulfonamide drug (sulfamethazine) and an aromatic amine carcinogen (2-aminofluorene) were observed for NAT2 variants possessing G191A (R64Q), T341C (I114T), A434C (E145P), G590A (R197Q), A845C (K282T) or G857A (G286T). Reductions in expression of NAT2 immunoreactive protein were observed for NAT2 variants possessing T341C, A434C or G590A. Reductions in protein stability were noted for NAT2 variants possessing G191A, A845C, G857A or, to some extent, G590A. No significant differences in mRNA expression or transformation efficiency were observed among any of the NAT2 alleles. These results suggest two mechanisms for slow acetylator phenotype(s) and more clearly define the effects of individual SNPs on human NAT2 expression, stability and catalytic activity.

Arylamine N-acetyltransferases - of mice, men and microorganisms

Arylamine N-acetyltransferases (NATs) catalyse the transfer of an acetyl group from acetyl CoA to the terminal nitrogen of hydrazine and arylamine drugs and carcinogens. These enzymes are polymorphic and have an important place in the history of pharmacogenetics, being first identified as responsible for the polymorphic inactivation of the anti-tubercular drug isoniazid. NAT has recently been identified within Mycobacterium tuberculosis itself and is an important candidate for modulating the response of mycobacteria to isoniazid. The first three-dimensional structure of the unique NAT family shows the active-site cysteine to be aligned with conserved histidine and aspartate residues to form a catalytic triad, thus providing an activation mechanism for transfer of the acetyl group from acetyl CoA to cysteine. The unique fold could allow different members of the NAT family to play a variety of roles in endogenous and xenobiotic metabolism.

[Prevention of renal carcinoma: the nutri-genetic approach]

The development of renal cell carcinoma (RCC) has been associated with both genetic and environmental factors, with somatic and germline mutations in the von Hippel-Lindau (VHL) tumor suppressor gene and with tobacco smoking, obesity, long term exposure to some nutrients, pollutants, and industrial solvents such as trichloroethylene. Intra and interfamilial variability of expression of germline mutations in the VHL gene and variable susceptibility to carcinogens in the sporadic forms strongly suggest the involvement of conditional modifier genes. In order to identify sub groups of individuals at increased risk because of susceptibility genotypes, we have collected a series of 460 patients who developed an RCC and 79 families with the von Hippel Lindau disease. To collect clinical and mutational data for correlation analysis we have developed a unique tool the Universal Mutation Database. Comparison of the spectrum of germline and somatic mutations in the VHL gene showed that: 1) in sporadic RCC mutations lead more often to truncated proteins (83%), while the remaining mutations (17%), include 3/4 of transversions and 1/4 of transitions. This high proportion of transversions supports the involvement of carcinogens the impact of which is conditioned by the genetic variability of xenobiotic metabolizing enzymes; 2) whereas in familial cases missense mutations are more common; this difference allowed us to define a prognostic factor for the occurrence of RCC in a VHL context. In order to look for genotypes conferring a higher risk we genotyped the RCC patients for 8 different genes (50 genotypes). A significant relationship was observed for several combinations of alleles including CYP1A1 ("variant"), NAT2 and NAT1 (slow) and GSTM1 (null allele). Associations between specific mutational profiles and at risk genotypes at different tumoral stages should allow us to: 1) define more precisely the nature of specific patterns of mutations in relation with the deficiency or overexpression of such or such enzymes in presence of particular carcinogens; 2) demonstrate that certain combinations of genotypes confer a particular risk to develop a specific type of tumor in VHL patients. Thus tracking of potentially carcinogenic substances, through their footprints and through identification of conditionally detrimental genotypes of genes participating in their detoxification should permit a better prevention through an appropriate nutrition adapted to each individual.

An update on genetic, structural and functional studies of arylamine N-acetyltransferases in eucaryotes and procaryotes

Arylamine N:-acetyltransferase (NAT) was first identified as the inactivator of the anti-tubercular drug isoniazid. The enzyme was shown to catalyse the transfer of an acetyl group from acetyl-CoA to the terminal nitrogen of the hydrazine drug. The rate of inactivation of isoniazid was polymorphically distributed in the population and was one of the first examples of pharmacogenetic variation. NAT was identified recently in Mycobacterium tuberculosis and is a candidate for modulating the response to isoniazid. Genome sequences have revealed many homologous members of this unique family of enzymes. The first three-dimensional structure of a member of the NAT family identifies a catalytic triad consisting of aspartate, histidine and cysteine proposed to form the activation mechanism. So far, all procaryotic NATs resemble the human enzyme which acetylates isoniazid (NAT2). Human NAT2 is characteristic of drug-metabolizing enzymes: it is found in liver and intestine. In humans and other mammals, there are up to three different isoenzymes. If only one isoenzyme is present, it is like human NAT1. Human NAT1 and its murine equivalent specifically acetylate the folate catabolite p-aminobenzoylglutamate. NAT1 and its murine homologue each have a ubiquitous tissue distribution and are expressed early in development at the blastocyst stage. During murine embryonic development, NAT is expressed in the developing neural tube. The proposed endogenous role of NAT in folate metabolism, and its multi-allelic nature, indicate that its role in development should be assessed further.

The putative tumour suppressor Fus-2 is an N-acetyltransferase

Acetyltransferases are essential enzymes for a wide variety of cellular processes and mutations in acetyltransferase genes have been associated with the development of certain cancers. For this reason, we conducted a computerized sequence homology search for novel acetyltransferases. Here, we show that the putative tumour suppressor protein Fus-2 has homology to the catalytic domain of acetyltransferases. We demonstrate that Fus-2 can acetylate the N-terminus of proteins using a ping-pong mechanism and that it has a specificity for substrates. Consistent with other N-acetyltransferases, Fus-2 localizes to the cytoplasm, as shown by GFP-tag experiments. Since the Fus-2 gene maps to the chromosomal region 3p21.3, which contains at least one tumour suppressor gene, the N-acetyltransferase functions of Fus-2 may be relevant to its potential role in cancer.

Metabolism of xenobiotics and chemical carcinogenesis

In order to avoid the accumulation of harmful xenobiotics in cells, living organisms have developed ways for their elimination. Multiple xenobiotic metabolizing enzymes with variable but partially overlapping catalytic properties play a key role in the elimination process. These enzymes are encoded by superfamilies of genes which, during the course of evolution, have evolved in a way that has made it possible for the different species to survive and take advantage of different habitats and diet containing a variable composition of harmful xenobiotics. As a result of this evolutionary process, species have achieved capacities to metabolize xenobiotics which are appropriate for their survival but which may differ considerably from those of other species. This evolutionary process may also explain the interethnic and interindividual variability of drug metabolism in humans. Because many carcinogens are substrates of drug-metabolizing enzymes it is reasonable to assume that humans have a variable capacity to activate or inactivate carcinogens. This has been shown to be the case. It appears that most of the carcinogen-metabolizing enzymes are inducible by xenobiotics: they respond to environmental stimuli and therefore vary in their activity. Furthermore, many of the encoding genes are polymorphic and multiple allelic variants relevant for the phenotype may exist in human populations. Analysis of the genetic variability that affects the capacity to metabolize carcinogens in humans has shown that a few members of the cytochrome P450, glutathione S-transferase and N-acetyltransferase gene families may play an Important role in chemical carcinogenesis. Yet for several enzymes such a role has not been established until now, although their catalytic properties and expression in human tissues suggest that such a role should exist. More studies on the role of individual enzymes in chemical carcinogenesis are therefore warranted.

Polymorphic NATs and cancer predisposition

The acetylation polymorphism, discovered 40 years ago, holds a special place as one of the first described examples of a pharmacogenetic defect affecting xenobiotic biotransformation capacity in human populations. The genetically determined N-acetyltransferase activity is involved in activation/inactivation reactions of numerous xenobiotics. Therefore, it has been suggested that slow acetylator status may modify the individual responses to various chemicals. In humans, two genes, NAT1 and NAT2, are responsible for N-acetyltransferase activity. To date several allelic variants of both NAT1 and NAT2 have been detected, and it has been suggested that some of them modify individual susceptibility to cancer. Slow NAT2 acetylation capacity has been suggested as conferring increased risk of bladder, breast, liver and lung cancers, and decreased risk of colon cancer, whereas a prominent change in the NAT1 gene, putatively associated with increased NAT1 activity, has been suggested as increasing the risk of bladder and colon cancer and decreasing that of lung cancer. While three of the NAT2 variants have been shown to account for most of the slow NAT2 acetylator genotypes in Caucasians, less complete data are available on how the NAT1 variants modify NAT1 activity in vivo. This review discusses present knowledge on NAT polymorphisms, particularly in relation to individual cancer predisposition.

Role of human N-acetyltransferases, NAT1 or NAT2, in genotoxicity of nitroarenes and aromatic amines in Salmonella typhimurium NM6001 and NM6002

Human NAT1 and NAT2 genes were subcloned into pACYC184 vector and the plasmids thus obtained were introduced into Salmonella typhimurium O-acetyltransferase-deficient strain NM6000 (TA1538/1, 8-DNP/pSK1002), establishing new strains NM6001 and NM6002, respectively. We compared the sensitivities of these two strains with those of NM6000 towards carcinogenic nitroarenes and aromatic amines in the SOS/umu response. The induction of umuC gene expression by these chemicals in the presence and absence of the S9 fraction was assayed by measuring the cellular beta-galactosidase activity expressed by the umuC"lacZ fusion gene in the tester strains. 2-Nitrofluorene and 2-aminofluorene induced umuC gene expression more strongly in the NM6001 strain than in the NM6002 strain. In contrast, induction of umuC gene expression by 1, 8-dinitropyrene, 6-aminochrysene and 2-amino-3,5-dimethylimidazo[4, 5-f]quinoline was weaker in the NM6001 strain than in the NM6002 strain. 1-Nitropyrene, 2-amino-6-methyl-dipyrido[1,2-a:3', 2'-d]imidazole, 3-amino-1,4-dimethyl-5H-pyrido[4,3-b]indole, 3-amino-1-methyl-5H-pyrido[4,3-b]indole, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine and 2-amino-3-methyl-9H-pyrido[2,3-b]indole were found to induce umuC gene expression at similar extents in both strains. These results suggest that the newly developed strains can be employed for the studies on mechanisms of genotoxicity of a variety of nitroarenes and aromatic amines, along with the assessment of cancer risk to humans.

[Biochemical, molecular genetic and ecogenetic studies of polymorphic arylamine N-acetyltransferase (NAT2) in the brain]

Polymorphic arylamine N-acetyltransferase (NAT2) is a detoxification enzyme in the liver. The activity of this enzyme differs for each individual. The difference of this enzyme activity is determined by the polymorphism of this enzyme which is known as N-acetylation polymorphism, and is a representative pharmacogenetic trait. The expression of NAT2 is also seen in brain tissue. However, its role has yet to be elucidated. To clarify the role of NAT2 in the brain, I prepared recombinant NAT2 and examined whether or not L-kynurenine could be an endogenous substrate of NAT2. The spectrophotometric and high performance liquid chromatography analyses of the product of the acetylation reaction using the recombinant enzyme revealed that no N-acetylkynurenine was produced by NAT2 from L-kynurenine. The topographical expression of NAT2 in the brain was examined by the in situ hybridization technique. Hybridization was done using seven digoxigenin-labeled oligonucleotide probes on fresh frozen sections and on paraffin embedded sections of adult rat brain tissue. Though strong signals were observed on the nerve cells and ependym cells, no signals were detected on the glial cells and blood vessels. This finding suggests that the nerve cell-specific endogenous substrate of NAT2 thus exists in the brain, and that the exogenous arylamines are metabolized in ependym. Furthermore, the ecogenetic association of NAT2 and Alzheimer disease (AD) was assessed using the polymerase chain reaction-based restriction fragment length polymorphism technique for NAT2 genotyping. These results revealed the frequency of rapid acetylator phenotype to increase in the non-apoE epsilon 4 carriers of late-onset AD (The odds ratio was 3.0, the 95% confidence interval was 1.3-7.3). This finding suggests that NAT2 polymorphism is one of the risk modifying factor for late-onset AD in non-apoE epsilon 4 carriers.

Ontogeny of a diurnal rhythm in arylalkylamine-N-acetyltransferase mRNA in rat pineal gland

Melatonin synthesis in the pineal gland of adult rats is linked to cAMP-dependent transcriptional and post-transcriptional regulatory mechanisms affecting its rate-limiting enzyme, the arylalkylamine-N-acetyltransferase (AA-NAT). During development of the pineal gland, neuronal control gains access to the earlier matured cAMP-signaling pathway to shape the day-night rhythm in AA-NAT enzymatic activity. By semiquantitative in situ hybridization we analyzed if the developmental onset of a rhythmic AA-NAT activity is correlated to a temporally parallel onset in AA-NAT transcription. We found that AA-NAT mRNA levels in rat pineal gland become rhythmic at postnatal day 5. Thus, AA-NAT gene transcription in rat pineal gland starts to show day-night differences shortly prior to the appearance of a rhythmic AA-NAT activity.

4-Aminobiphenyl-DNA adducts and p53 mutations in bladder cancer

Epidemiologic studies have suggested that smokers of air-cured tobacco (rich in arylamines) are at higher risk of bladder cancer than smokers of flue-cured tobacco. The risk has been shown to be modulated by the N-acetyltransferase genotype. We analyzed the biopsies of 45 patients with bladder cancer. p53 mutations were sought by direct sequencing, and 4-aminobiphenyl-DNA adducts were measured by negative ion gas chromatography-mass spectrometry. 4-Aminobiphenyl-DNA adducts were higher in smokers of air-cured tobacco and in current smokers, but no relationship with the number of cigarettes smoked was found. Adducts were higher in more advanced histologic grades of tumors. No pattern was evident for p53 mutations. Seven of 9 mutations occurred in grade 3 tumors. No association was found between 4-ABP adducts and GSTM1 or NAT2 genetic polymorphisms.

Cigarette smoking, N-acetyltransferase 2 genetic polymorphisms, and breast cancer risk

OBJECTIVE

To determine if N-acetyltransferase 2 (NAT2) polymorphisms result in decreased capacity to detoxify carcinogenic aromatic amines in cigarette smoke, thus making some women who smoke more susceptible to breast cancer.

DESIGN

Case-control study with genetic analyses. DNA analyses were performed for 3 polymorphisms accounting for 90% to 95% of the slow acetylation phenotype among whites.

SETTING AND PARTICIPANTS

White women with incident primary breast cancer (n=304) and community controls (n=327).

RESULTS

Neither smoking nor NAT2 status was independently associated with breast cancer risk. There were no clear patterns of increased risk associated with smoking by NAT2 status among premenopausal women. In postmenopausal women, NAT2 strongly modified the association of smoking with risk. For slow acetylators, current smoking and smoking in the distant past increased breast cancer risk in a dose-dependent manner (odds ratios [95% confidence intervals] for the highest quartile of cigarettes smoked 2 and 20 years previously, 4.4 [1.3-14.8] and 3.9 [1.4-10.8], respectively). Among rapid acetylators, smoking was not associated with increased breast cancer risk.

CONCLUSIONS

Our results suggest that smoking may be an important risk factor for breast cancer among postmenopausal women who are slow acetylators, demonstrate heterogeneity in response to carcinogenic exposures, and may explain previous inconsistent findings for cigarette smoking as a breast cancer risk factor.

Role of aromatic amine acetyltransferases, NAT1 and NAT2, in carcinogen-DNA adduct formation in the human urinary bladder

The metabolic activation and detoxification pathways associated with the carcinogenic aromatic amines provide an extraordinary model of polymorphisms that can modulate human urinary bladder carcinogenesis. In this study, the metabolic N-acetylation of p-aminobenzoic acid (PABA) to N-acetyl-PABA (NAT1 activity) and of sulfamethazine (SMZ) to N-acetyl-SMZ (NAT2 activity), as well as the O-acetylation of N-hydroxy-4-aminobiphenyl (OAT activity; catalyzed by NAT1 and NAT2), were measured in tissue cytosols prepared from 26 different human bladder samples; then DNA was isolated for determination of NAT1 and NAT2 genotype and for analyses of carcinogen-DNA adducts. Both PABA and OAT activities were detected, with mean activities +/- SD of 2.9 +/- 2.3 nmol/min/mg protein and 1.4 +/- 0.7 pmol bound/mg DNA/min/mg protein, respectively. However, SMZ activities were below the assay limits of detection (< 10 pmol/min/mg protein). The levels of putative carcinogen-DNA adducts were quantified by 32P-postlabeling and averaged 2.34 +/- 2.09 adducts/10(8) deoxyribonucleotide phosphate (dNp). Moreover, the DNA adduct levels in these tissues correlated with their NAT1-dependent PABA activities (r = 0.52; P < 0.01) but not with their OAT activities. Statistical and probit analyses indicated that this NAT1 activity was not normally distributed and appeared bimodal. Applying the NAT1:OAT activity ratios (N:O ratio) allowed arbitrary designation of rapid and slow NAT1 phenotypes, with a cutpoint near the median value. Within each of these subgroups, NAT1 correlated with OAT (P < 0.05); DNA adduct levels were elevated 2-fold in individuals with the rapid NAT1 or NAT1/OAT phenotype. Examination of DNA sequence polymorphisms in the NAT1 gene by PCR have demonstrated that an NAT1 polyadenylation polymorphism is associated with differences in tissue NAT1 enzyme activity; accordingly, NAT1 activity in the bladder of individuals with the heterozygous NAT1*10 allele was 2-fold higher than in subjects homozygous for the putative wild-type NAT1*4 allele. Likewise, DNA adduct levels in the mucosa of the urinary bladder were found to be 2-fold (P < 0.05) higher in individuals with the heterozygous NAT1*10 allele (3.5 +/- 2.1 adducts/10(8) dNp) as compared to NAT1*4 homozygous (1.8 +/- 1.9 adducts/10(8) dNp). Thus, these data provide strong support for the hypothesis that NAT1 activity in the urinary bladder mucosa represents a major bioactivation step that converts urinary N-hydroxy arylamines to reactive N-acetoxy esters that form covalent DNA adducts.(ABSTRACT TRUNCATED AT 400 WORDS)

Properties of clock-controlled and constitutive N-acetyltransferases from chick pineal cells

The pineal gland synthesizes its hormone melatonin (O-methyl-N-acetylserotonin) from serotonin. Acetyl-CoA: serotonin N-acetyltransferase (SNAT), the enzyme that catalyzes the committed step in this biosynthesis, is largely restricted to the pineal gland and is regulated by adrenergic and circadian mechanisms. Another enzyme, acetyl-CoA: arylamine N-acetyltransferase (ANAT), having an apparently similar activity, is also present in the pineal. This enzyme, however, is not rhythmically regulated. SNAT activity of cultured chick pineal cells was obtained without ANAT after ammonium sulfate precipitation. ANAT activity was retained without SNAT activity after pre-incubation at 37 degrees C. Thus, each enzyme could be examined independently. Overlap in substrate specificity between the two enzymes was minimal. Kinetic analysis of the separated enzyme activities revealed that while SNAT operates via a random or ordered bi bi mechanism, ANAT catalysis occurs through a ping pong bi bi mechanism with substrate inhibition by acetyl-CoA. By size-exclusion chromatography, ANAT was confirmed to be 30-35 kDa, and SNAT was estimated at 15-20 kDa. Taken together, these results indicate that the two enzymes differ in their structure, reactivity, stability, and mechanism of catalysis.

Serotonin N-acetyltransferase activity as a target for temperature in the regulation of melatonin production by frog retina

The adaptive mechanisms of serotonin N-acetyltransferase (NAT) activity in the regulation of melatonin synthesis in frog retina in the face of chronic and acute temperature changes have been investigated. We performed thermal acclimation experiments to test different environmental temperatures at two seasons of the year (summer and winter), followed by the set-up of an eyecup culture system to investigate the acute effects of temperature on NAT activity and melatonin production daily rhythms. Low temperature induced a significant increase in NAT activity, independent of both the time of the photocycle (midday or midnight) and the season of the year (winter or summer). Acute cold-induced stimulation of NAT activity may be associated with lower decreases in the enzyme synthesis rate, rather than decreases in the degradation rate. In contrast, acclimation to warm temperature (25 degrees C) stimulated ocular melatonin production. Nocturnal melatonin production in eyecups cultured at 25 degrees C was significantly higher than in eyecups cultured at 5 degrees C. We suggest that this discrepancy in thermal regulation of melatonin synthesis can be justified by a seasonal variation in serotonin content within the photoreceptor cells, which determines the thermal response of melatonin production through changes in NAT kinetics.

Cloning, expression, and functional characterization of two mutant (NAT2(191) and NAT2(341/803)) and wild-type human polymorphic N-acetyltransferase (NAT2) alleles

The N-acetylation polymorphism segregates individuals into rapid, intermediate, and slow acetylator phenotypes via monogenic inheritance at the NAT2 locus. In a previous study (Arch. Toxicol. 67, 445-452, 1993), we uncovered discrepancies between apparent NAT2 acetylator genotype based on polymerase chain reaction-restriction fragment length polymorphism analysis, in vitro colon arylamine N-acetyltransferase activity, and expected frequency of slow acetylator phenotype in African-Americans, which suggested the presence of not yet defined mutant NAT2 alleles. Two novel NAT2 alleles were discovered after cloning and sequencing of NAT2 polymerase chain reaction products. One allele (NAT2(191)) contained a point mutation at nucleotide 191 [G-->A (Arg-->Gln)], whereas the other allele (NAT2(341/803)) contained two point mutations [341T-->C (Ile-->Thr); 803A-->G (Lys-->Arg)]. The two mutant NAT2 and the NAT2wt alleles were expressed in a prokaryotic expression system. Both the NAT2(191) and NAT2(341/803) mutant alleles expressed functional N-acetyltransferases capable of catalyzing both arylamine N-acetylation and the metabolic activation (via O-acetylation) of N-hydroxy-2-aminofluorene. However, the NAT2(191) and NAT2(341/803) each exhibited significantly lower N- and O-acetylation capacity and were intrinsically less stable than NAT2wt.

Construction of Syrian hamster lines congenic at the polymorphic acetyltransferase locus (NAT2): acetylator genotype-dependent N- and O-acetylation of arylamine carcinogens

Congenic Bio. 1.5/H-NAT2 Syrian hamster lines were constructed by introducing the NAT2r gene from MHA/SsLak inbred hamsters into a background BIO 1.5 Syrian inbred hamster line. Genetic identity of the Bio. 1.5/H-NAT2 congenic lines and nonidentity with the previously constructed Bio. 82.73/H-Pat congenic lines were determined by "DNA fingerprints" of genomic DNA derived from the different hamster lines. The N-acetylation capacity of the Bio. 1.5/H-NAT2 congenic hamster lines was clearly NAT2-dependent both in vivo and in vitro, with highest levels expressed in Bio. 1.5/H-NAT2r homozygous rapid acetylators, intermediate levels in Bio. 1.5/H-NAT2r/NAT2s heterozygous acetylators, and lowest levels in Bio. 1.5/H-NAT2s homozygous slow acetylators. The NAT2-dependent expression of N-acetyltransferase activity was evident toward p-aminobenzoic acid, 4-aminophenol, 2-aminofluorene, 4-aminobiphenyl, beta-naphthylamine, and 3,2'-dimethyl-4-amino-biphenyl in liver, kidney, colon, lung, and urinary bladder cytosols. The polymorphic acetyltransferase (NAT2) and the monomorphic acetyltransferase (NAT1) were isolated from hepatic cytosols and tested separately for their ability to catalyze arylamine N-acetyltransferase and N-hydroxyarylamine O-acetyltransferase activities. Both arylamine N-acetylation and N-hydroxyarylamine O-acetylation were clearly acetylator genotype-dependent when catalyzed by NAT2, and both were clearly acetylator genotype-independent when catalyzed by NAT1. NAT2/NAT1 activity ratios varied with the particular arylamine substrate acetylated. These studies show an important role for NAT2 acetylator genotype in Syrian hamster carcinogenic arylamine metabolism and confirm its role in the metabolic activation of N-hydroxyarylamines. The Bio. 1.5/H-NAT2 congenic lines provide a new model for investigating the precise role of the NAT2 gene locus in arylamine metabolism and toxicity.

Circadian rhythm in pineal N-acetyltransferase activity: phase shifting by dark pulses (III)

N-Acetyltransferase (NAT) is an enzyme whose rhythmic activity in the pineal gland and retina is thought to be responsible for melatonin circadian rhythms. The enzyme has circadian properties--its rhythm persists in constant conditions, and it is precisely controlled by light and dark. Experiments are reported in which 4-h light or dark pulses were imposed on chicks (Gallus domesticus) over a 24-h period. Pineal NAT profiles were measured during and subsequent to the pulses. The phase of the NAT cycle following pulses was plotted to obtain phase-response curves. Light pulses produced a maximum phase shift (advance of 5 h) 8 h after the expected time of lights-out; dark pulses produced a maximum phase shift (advance of 4 h) 3 h after the expected time of lights-out. Maximum phase delays (-2 h) occurred 1-2 h after the expected lights-out for light pulses and 8 h after expected lights-on for dark pulses.