Effects of ions on adenosine binding and enzyme activity of purified S-adenosylhomocysteine hydrolase from bovine kidney

Analyzing S-adenosylhomocysteine hydrolase gene sequences in deuterostome genomes

S-adenosylhomocysteine hydrolase (SAHH) gene sequences of sea-urchin, two amphioxus, sea-squirt and eight vertebrates are comparatively analyzed in the current analysis. Although SAHH protein sequences are highly conserved in these species, their nucleotide sequences are much different, ranging from 5,446 bp in amphioxus to 40,174 bp in zebra fish. The length divergence is mainly caused by distinct introns in some species. SAHH genes in amphioxus (or sea-urchin), sea-squirt and vertebrates are composed of eight, nine and ten exons, respectively. Sequence alignment shows that exon 3 in amphioxus and sea-urchin is similar to exons 3 + 4 in vertebrates, exon 5 in amphioxus and sea-urchin is similar to exons 5 + 6 in sea-squirt, and the two exons are fused into exon 6 in vertebrates. Furthermore, exon 7 in sea-squirt is similar to exons 7 + 8 in vertebrates, indicating that exon-fission and exon-fusion events have been taken place during the evolution of deuterostome SAHH genes. Active sites and NAD+-binding sites are located in exons 2 7 in amphioxus, which are dispersed into much more exons along with the evolution of vertebrates. It is speculated that ten-exon organization of SAHH gene occurred after the separation of invertebrates and vertebrates. Synonymous and non-synonymous substitution analysis shows that negative selection plays a dominant role in the evolution of SAHH genes. Phylogenetic analysis shows that SAHH genes in amphioxus, sea-urchin and sea-squirt form a cluster and locate at the base of neighbor-joining tree, suggesting that they are the archetype of vertebrate SAHH genes.

DYRK1A, a novel determinant of the methionine-homocysteine cycle in different mouse models overexpressing this Down-syndrome-associated kinase

BACKGROUND

Hyperhomocysteinemia, characterized by increased plasma homocysteine level, is associated with an increased risk of atherosclerosis. On the contrary, patients with Down syndrome appear to be protected from the development of atherosclerosis. We previously found a deleterious effect of hyperhomocysteinemia on expression of DYRK1A, a Down-syndrome-associated kinase. As increased expression of DYRK1A and low plasma homocysteine level have been associated with Down syndrome, we aimed to analyze the effect of its over-expression on homocysteine metabolism in mice.

METHODOLOGY/PRINCIPAL FINDINGS

Effects of DYRK1A over-expression were examined by biochemical analysis of methionine metabolites, real-time quantitative reverse-transcription polymerase chain reaction, and enzyme activities. We found that over-expression of Dyrk1a increased the hepatic NAD(P)H:quinone oxidoreductase and S-adenosylhomocysteine hydrolase activities, concomitant with decreased level of plasma homocysteine in three mice models overexpressing Dyrk1a. Moreover, these effects were abolished by treatment with harmine, the most potent and specific inhibitor of Dyrk1a. The increased NAD(P)H:quinone oxidoreductase and S-adenosylhomocysteine hydrolase activities were also found in lymphoblastoid cell lines from patients with Down syndrome.

CONCLUSIONS/SIGNIFICANCE

Our results might give clues to understand the protective effect of Down syndrome against vascular defect through a decrease of homocysteine level by DYRK1A over-expression. They reveal a link between the Dyrk1a signaling pathway and the homocysteine cycle.

Characterization, expression and localization of S-adenosylhomocysteine hydrolase from amphioxus Branchiostoma belcheri tsingtaunese

A cDNA clone encoding AmphiSAHH [amphioxus SAHH (S-adenosylhomocysteine hydrolase)] protein was isolated from a cDNA library from the gut of Branchiostoma belcheri tsingtaunese. It contained a 1305 bp open reading frame corresponding to a deduced protein of 434 amino acid residues, with a predicted molecular mass of approx. 47.8 kDa. Phylogenetic analysis showed that AmphiSAHH and sea-urchin SAHH joined together and positioned at the base of the vertebrate SAHH clade, suggesting that both AmphiSAHH and sea-urchin SAHH might share some characteristics of the archetype of vertebrate SAHH proteins. The genomic DNA sequence of AmphiSAHH contained eight exons and seven introns, which was similar to B. floridae and sea-urchin SAHH exon/intron organization. Sequence comparison suggested the evolutionary appearance of the ten exon/nine intron organization of SAHH genes after the split of invertebrates and vertebrates, after which it has been highly conserved. AmphiSAHH has been successfully expressed in Escherichia coli and purified. Western blotting confirmed that the enzyme has a native molecular mass of approx. 48 kDa, and the catalytic activities and NAD(+)/NADH binding affinity of recombinant AmphiSAHH were measured. Immunohistochemistry analysis showed that SAHH was strongly expressed in hepatic caecum, gill, spermary and ovary of amphioxus.

Determinants for the cAMP-binding site at the S-adenosylhomocysteine-hydrolase

S-Adenosylhomocysteine-hydrolase (AdoHcy-hydrolase) catalyzes the reversible hydrolysis of S-adenosylhomocysteine (AdoHcy) to adenosine (Ado) and homocysteine (Hcy). Since Ado competes with cAMP at the high affinity-binding site of the enzyme, we determined the effect of cAMP on enzyme activity and its binding characteristics to purified AdoHcy-hydrolase from bovine kidney in its native, in its fully oxidized (NAD(+)), and in its fully reduced (NADH) form. cAMP (10 micromol/l) enhanced the hydrolytic activity of native AdoHcy-hydrolase by 35%, whereas the activity of the enzyme in its NAD(+) form was not stimulated by cAMP. In contrast to azido-Ado, binding of azido-cAMP did not inhibit the enzymatic activity of AdoHcy-hydrolase. Furthermore, cAMP did not prevent the Ado induced inhibition of the AdoHcy hydrolysis. Saturation binding experiments with the three different forms of AdoHcy-hydrolase, native, NAD(+), and NADH showed only one binding site with high affinity. This binding site was identified after photoaffinity labeling of the enzyme with 8-azido-[2-(3)H]-cAMP. One photolabeled peptide was isolated as Trp(310)-Val(325) from each AdoHcy-hydrolase from native, NAD(+), and NADH. The cAMP-labeled peptide is located in the NAD-binding domain of AdoHcy-hydrolase. In conclusion, our data show that the cAMP-binding site at the AdoHcy-hydrolase is independent of the NAD(+)/NADH ratio of the enzyme and is identical with the high affinity-binding site of Ado. Moreover, cAMP did not interact with the catalytic site of AdoHcy-hydrolase and did not act as an allosteric effector for the AdoHcy-hydrolase.

Effect of vitamin B6 deficiency on S-adenosylhomocysteine hydrolase activity as a target point for methionine metabolic regulation

The objective of this study was to clarify the relationship between the accumulation of S-adenosylhomocysteine (SAH) and the change in the SAH hydrolase activity in vitamin B6 (B6). Male Wistar rats were fed a control diet (control and pair-fed groups) or B6-free diet (B6-deficient group) for 5 wk. Although the SAH-synthetic activity of SAH hydrolase significantly increased in the B6-deficient group, SAH-hydrolytic activity of SAH hydrolase showed no significant difference in the liver among the three groups. On the other hand, SAH hydrolase mRNA in the liver did not show any significant change. Thus, the accumulation of SAH would be due to the increased SAH-synthetic activity of SAH hydrolase. The disturbed methionine metabolism by B6-deficiency, such as a significant increase of plasma homocysteine, might induce the activation of SAH hydrolase in the direction of SAH synthesis.

Importance of the trans-sulfuration pathway in cancer prevention and promotion

The trans-sulfuration pathway is a biochemical mechanism that links methionine metabolism to the biosynthesis of cellular redox-controlling molecules, like cysteine, glutathione, and taurine. While there is some knowledge about the metabolic intermediates and enzymes that participate in trans-sulfuration, little is known about the physiological importance of this mechanism. Deficiencies within the trans-sulfuration pathway induces (i) the generation of reactive species of oxygen (ROS) and halogens (RHS), (ii) homocyst(e)ine accumulation, and (iii) the synthesis of proinflammatory molecules by macrophages, and contribute to humans pathologies like atherosclerosis and tumor development. In this review we outline the role of this biochemical pathway in tumor development and analyze current findings on the role of trans-sulfuration in mammalian physiology. The potential relationship between chronic inflammation, and tumor and atherosclerotic development are discussed.

Adenosine and Kidney Function

In this review we outline the unique effects of the autacoid adenosine in the kidney. Adenosine is present in the cytosol of renal cells and in the extracellular space of normoxic kidneys. Extracellular adenosine can derive from cellular adenosine release or extracellular breakdown of ATP, AMP, or cAMP. It is generated at enhanced rates when tubular NaCl reabsorption and thus transport work increase or when hypoxia is induced. Extracellular adenosine acts on adenosine receptor subtypes in the cell membranes to affect vascular and tubular functions. Adenosine lowers glomerular filtration rate (GFR) by constricting afferent arterioles, especially in superficial nephrons, and acts as a mediator of the tubuloglomerular feedback, i.e., a mechanism that coordinates GFR and tubular transport. In contrast, it leads to vasodilation in deep cortex and medulla. Moreover, adenosine tonically inhibits the renal release of renin and stimulates NaCl transport in the cortical proximal tubule but inhibits it in medullary segments including the medullary thick ascending limb. These differential effects of adenosine are subsequently analyzed in a more integrative way in the context of intrarenal metabolic regulation of kidney function, and potential pathophysiological consequences are outlined.

Synthesis of the sulfur amino acids: cysteine and methionine

This review will assess new features reported for the molecular and biochemical aspects of cysteine and methionine biosynthesis in Arabidopsis thaliana with regards to early published data from other taxa including crop plants and bacteria (Escherichia coli as a model). By contrast to bacteria and fungi, plant cells present a complex organization, in which the sulfur network takes place in multiple sites. Particularly, the impact of sulfur amino-acid biosynthesis compartmentalization will be addressed in respect to localization of sulfur reduction. To this end, the review will focus on regulation of sulfate reduction by synthesis of cysteine through the cysteine synthase complex and the synthesis of methionine and its derivatives. Finally, regulatory aspects of sulfur amino-acid biosynthesis will be explored with regards to interlacing processes such as photosynthesis, carbon and nitrogen assimilation.

Effect of acute hyperhomocysteinemia on methylation potential of erythrocytes and on DNA methylation of lymphocytes in healthy male volunteers

Homocysteine is a precursor of S-adenosylmethionine (AdoMet) and a metabolite of S-adenosylhomocysteine (AdoHcy). The ratio of AdoMet to AdoHcy, defined as the methylation potential (MP), indicates the flow of methyl groups within the cells. Chronic elevations of total homocysteine (tHcy) in plasma correlate with increased AdoHcy concentrations, decreased MP, and impaired DNA methylation. However, the influence of acute hyperhomocysteinemia on MP is unknown. We induced acute hyperhomocysteinemia in 14 healthy volunteers by oral administration of l-homocysteine (65.1 μmol/kg body wt) in an open, randomized, placebo-controlled two-period crossover study. The kinetics of tHcy in blood and urine, MP in blood, and global DNA methylation in lymphocytes were studied systematically during 48 h. Plasma tHcy concentrations reached a peak at 34 ± 11 min after an oral load with l-homocysteine and decreased with a half-life of 257 ± 41 min (means ± SD). Only 2.3% of the homocysteine dose were recovered in urine. AdoHcy concentrations and MP in whole blood and erythrocytes were not affected by the oral homocysteine load. Furthermore, global DNA methylation in lymphocytes did not change under these conditions. We found no difference between the genotypes of 5,10-methylenetetrahydrofolate reductase in response to the homocysteine load. However, AdoMet content in erythrocytes was significantly higher in the C677T carriers (CT; n = 7) compared with the CC genotype ( n = 7). Although chronic elevation of tHcy has been shown to affect MP and DNA methylation, acute elevation of plasma tHcy above 20 μmol/l for 8 h is not sufficient to change MP and to induce DNA hypomethylation in lymphocytes.

S-Adenosylhomocysteine hydrolase as a target for intracellular adenosine action

S-Adenosylhomocysteine hydrolase (AdoHcyase) controls intracellular levels of S-adenosylhomocysteine (AdoHcy). AdoHcy is a potent product inhibitor of some S-adenosylmethionine-dependent methyltransferases. Pharmacological modulation of AdoHcyase to indirectly inhibit methyltransferases can be guided by the fact that adenosine binds with high affinity to AdoHcyase and inhibits enzyme activity. cAMP can compete with adenosine and can counteract the adenosine-induced inhibition of AdoHcyase. Thus, the ratio between adenosine and cAMP, which can vary under different physiological conditions, might result in changes in, for example, DNA promoter methylation and therefore transcription.

Hepatic transmethylation reactions in micropigs with alcoholic liver disease

Alcoholic liver disease is associated with abnormal hepatic methionine metabolism, including increased levels of homocysteine and S-adenosylhomocysteine (SAH) and reduced levels of S-adenosylmethionine (SAM) and glutathione (GSH). The concept that abnormal methionine metabolism is involved in the pathogenesis of alcoholic liver disease was strengthened by our previous findings in a micropig model where combining dietary folate deficiency with chronic ethanol feeding produced maximal changes in these metabolites together with early onset of microscopic steatohepatitis and an eightfold increase in plasma aspartate aminotransferase. The goal of the present study was to determine potential mechanisms for abnormal levels of these methionine metabolites by analyzing the transcripts and activities of transmethylation enzymes in the livers of the same micropigs. Ethanol feeding or folate deficiency, separately or in combination, decreased transcript levels of methylenetetrahydrofolate reductase (MTHFR), methionine adenosyltransferase (MAT1A), glycine-N-methyltransferase (GNMT) and S-adenosylhomocysteine hydrolase (SAHH). Ethanol feeding alone reduced the activities of methionine synthase (MS) and MATIII and increased the activity of GNMT. Each diet, separately or in combination, decreased the activities of MTHFR and SAHH. In conclusion, the observed abnormal levels of methionine metabolites in this animal model of accelerated alcoholic liver injury can be ascribed to specific effects of ethanol with or without folate deficiency on the expressions and activities of hepatic enzymes that regulate transmethylation reactions. These novel effects on transmethylation reactions may be implicated in the pathogenesis of alcoholic liver disease.

Influence of an altered methylation potential on mRNA methylation and gene expression in HepG2 cells

S-adenosylhomocysteine (AdoHcy), a by-product and inhibitor of S-adenosylmethionine (AdoMet)-dependent methylation reactions, is removed by AdoHcy hydrolase. The ratio of AdoMet and AdoHcy, also termed methylation potential (MP), is a metabolic indicator for cellular methylation status. In the present study, we have investigated the influence of hypoxia and inhibition of AdoHcy hydrolase on MP in HepG2 cells. Furthermore, we studied the impact of deviations in MP on mRNA and DNA methylation and the expression of selected genes: erythropoietin, VEGF-A, AdoHcy hydrolase, cyclophilin, and HIF-1alpha. Under hypoxic conditions, the MP raised from 53.4 +/- 3.3 to 239.4 +/- 24.8, which is the result of increased AdoMet and decreased AdoHcy levels. Inhibition of AdoHcy hydrolase by adenosine-2',3'-dialdehyde leads to a 40-fold reduction of the MP under both normoxic and hypoxic conditions. Hypoxia increases erythropoietin (2.7-fold) and VEGF-A (5-fold) mRNA expression. During a reduced MP erythropoietin mRNA expression is lowered under normoxia and hypoxia by 70%, whereas VEGF-A mRNA expression is only reduced under hypoxic conditions by 60%. The mRNA expression of AdoHcy hydrolase, HIF-1alpha, and cyclophilin is insensitive to an altered MP. Furthermore, decreased MP leads to a highly significant decrease in overall mRNA methylation. Our results show that the mRNA levels of the studied genes respond differentially to changes in MP. This implies that genes with a slower transcription rate and mRNAs with a slower turnover are insensitive to short-term changes in MP.

Adenosine binding sites at S-adenosylhomocysteine hydrolase are controlled by the NAD+/NADH ratio of the enzyme

S-Adenosylhomocysteine hydrolase (AdoHcy hydrolase) catalyzes the reversible hydrolysis of S-adenosylhomocysteine (AdoHcy) to adenosine (Ado) and homocysteine. On the basis of the kinetics of Ado binding to AdoHcy hydrolase we have shown that AdoHcy hydrolase binds Ado with different affinities [Kidney Blood Press. Res. 19 (1996) 100]. Since AdoHcy hydrolase in its totally reduced form binds Ado with high affinity we determined in the present study the Ado binding characteristics of purified AdoHcy hydrolase from bovine kidney (native form) and of reconstituted forms with defined NAD(+)/NADH ratios. AdoHcy hydrolase in its native form and at a ratio of 50% NAD(+) and 50% NADH exhibits two binding sites for Ado with a K(D1) of 9.2+/-0.6 nmol/L and a K(D2) of 1.4+/-0.1 micromol/L, respectively. Binding of Ado to AdoHcy hydrolase in its NADH form and in its NAD(+) form exhibits only one binding site with high affinity 48.3+/-2.7 nmol/L for the NADH form and with a low affinity of 4.9+/-0.3 micromol/L for the NAD(+) form. To identify these two Ado binding sites, AdoHcy hydrolase was covalently modified with [2-3H]-8-azido-Ado. After irradiation of the native AdoHcy hydrolase two different photolabeled peptides were isolated and identified as Asp(307)-Val(325) and Tyr(379)-Thr(410). When the reconstituted AdoHcy hydrolase in its NADH and in its NAD(+) form was irradiated with [2-3H]-8-azido-Ado only one peptide was identified as Asn(312)-Lys(318) from the NADH form and as Asp(391)-Ala(396) from the NAD(+) form. Based on the crystallographic data, the labeled peptide Asp(391)-Ala(396) (low affinity binding site), appears to belong to the catalytic domain of AdoHcy hydrolase, whereas the labeled peptide, identified as Asn(312)-Lys(318) (high affinity binding site), is located in the NAD domain. In conclusion, our data show that AdoHcy hydrolase has two different Ado binding sites which are dependent upon the enzyme-bound NAD(+)/NADH ratios.

Characterization of the cAMP binding site of purified S-adenosyl-homocysteine hydrolase from bovine kidney

The enzyme S-adenosyl-homocysteine hydrolase (AdoHcyase) which catalyzes the reversible hydrolysis of AdoHcy to adenosine and homocysteine is an adenosine binding protein. In the present study we examined the characteristics of [(3)H]cAMP binding to purified AdoHcyase from bovine kidney in comparison with the high affinity adenosine binding site of AdoHcyase. AdoHcyase exhibits one [(3)H]cAMP binding site with an affinity of K(d)=23.1+/-1.1nM and a B(max) of 116.6+/-3.8pmol/mg protein. Binding of [(3)H]cAMP obeyed a monophasic reaction with a k(+1) value of 0.035min/M. The dissociation of AdoHcyase-[(3)H]cAMP complex exhibited a time- and temperature-dependent character. After a 240min incubation at 0 degrees only 5-10%, however, at 20 degrees 90% were displaceable. Adenosine and cAMP displace each other with similar affinities of EC(50) 57nM vs. EC(50) 65nM. 2'-Deoxyadenosine, N(6)-methyladenosine, and NECA displace 25nM [(3)H]cAMP and 10nM [(3)H]adenosine with EC(50) values of 94, 90 and 80nM, respectively. All other nucleosides studied, adenine, inosine, adenosine-2',3'-dialdehyde, 2-chloroadenosine, aristeromycin, and adenine nucleotides were only week competitors for [(3)H]cAMP and [(3)H]adenosine. These compounds displace [(3)H]cAMP and [(3)H]adenosine with equal potencies. Our data indicate that the binding site for nanomolar concentrations of cAMP and adenosine at the AdoHcyase appears to be identical. The physiological implications of a cAMP binding site at the AdoHcyase remain to be established.

Effect of hyperhomocysteinemia on plasma or tissue adenosine levels and renal function

BACKGROUND

Hyperhomocysteinemia (hHcys) is considered an independent risk factor of cardiovascular diseases. Recent studies in our laboratory have shown that hHcys produced glomerular dysfunction and sclerosis independently of hypertension. However, the mechanism mediating these pathogenic effects of homocysteine (Hcys) is poorly understood. Because Hcys and adenosine (Ado) are simultaneously produced via hydrolysis of S-adenosylhomocysteine (SAH), we hypothesized that hHcys may produce its pathogenic effects by decrease in plasma or tissue Ado concentrations.

METHODS AND RESULTS

L-Hcys (1.5 micromol/min per kilogram) was infused intravenously for 60 minutes to produce acute hHcys in Sprague-Dawley rats. Plasma Hcys levels increased from 6.7+/-0.4 to 14.7+/-0.5 micromol/L, but Ado decreased from 141.7+/-15.1 to 52.4+/-6.8 nmol/L in these rats with acute hHcys. This hHcys-induced reduction of Ado was also observed in the kidney dialysate. In rats with chronic hHcys, plasma Ado levels were also significantly decreased. By kinetic analysis of the enzyme activities, decrease in renal Ado levels in hHcys was shown to be associated with inhibition of SAH hydrolase but not 5'-nucleotidase. Functionally, intravenous infusion of Hcys was found to decrease renal blood flow, glomerular filtration rate, and sodium and water excretion, which could be blocked by the Ado receptor antagonist 8-SPT.

CONCLUSIONS

These results strongly suggest that hHcys decreases plasma and tissue Ado concentrations associated with inhibition of SAH hydrolase. Decrease in plasma and tissue Ado may be an important mechanism mediating the pathogenic effects of Hcys.

Tissue levels of S-adenosylhomocysteine in the rat kidney: effects of ischemia and homocysteine

Most S-adenosylmethionine (AdoMet)-dependent methyltransferases are regulated in vivo by the AdoMet/S-adenosylhomocysteine (AdoHcy) ratio, also termed as "methylation potential." Since adenosine inhibits in vitro AdoHcy hydrolysis and since adenosine tissue levels increase during hypoxia, it can be predicted that AdoHcy levels may increase in the rat kidney in parallel of those of adenosine. Therefore, the present investigation was performed to assess changes of renal AdoHcy and AdoMet tissue contents during ischemia and after administration of adenosine and homocysteine or both in the ischemic rat kidney. In anesthetized rats ischemia of the kidney was induced by renal artery occlusion for various time intervals. Adenosine and homocysteine were infused into the renal artery of the ischemic kidney. To induce a hyperhomocysteinemia homocysteine was continuously infused. The kidneys were removed and immediately snap-frozen. Tissue contents of AdoHcy, AdoMet, adenosine and adenine nucleotides were analyzed by means of HPLC. Under normoxic condition the tissue contents of AdoHcy, AdoMet and adenosine were 0.7+/-0.05, 44.1+/-1.0 and 3.8+/-0.1nmol/g wet weight, respectively. Renal ischemia for 30min resulted in an increase of AdoHcy levels from 0.7+/-0.05 to 9.1+/-0.6nmol/g wet weight and in a dramatic decrease of the AdoMet/AdoHcy ratio and energy charge from 65.1+/-5.6 to 2.8+/-0.2 and from 0.87+/-0.01 to 0.25+/-0.01, respectively. Application of exogenous adenosine into the ischemic kidney did not result in further AdoHcy accumulation. However, when homocysteine was infused into the ischemic kidney, AdoHcy increased five-fold above control levels, during 5min ischemia. Systemic infusion of homocysteine leads to a reduction of the methylation potential also in the normoxic kidney. We conclude that (i) the methylation potential in the kidney is markedly reduced during ischemia, mainly due to accumulation of AdoHcy; (ii) elevation of AdoHcy tissue content during ischemia is the result of the inhibition of AdoHcy hydrolysis; (iii) homocysteine is rate limiting for AdoHcy synthesis in the ischemic kidney; (iv) under normoxic conditions hyperhomocysteinemia can affect the methylation potential in the renal tissue.

Localization of S-adenosylhomocysteine hydrolase in the rat kidney

S-adenosylhomocysteine (SAH) hydrolase is a cytosolic enzyme present in the kidney. Enzyme activities of SAH hydrolase were measured in the kidney in isolated glomeruli and tubules. SAH hydrolase activity was 0.62 +/- 0.02 mU/mg in the kidney, 0.32 +/- 0.03 mU/mg in the glomeruli, and 0.50 +/- 0.02 mU/mg in isolated tubules. Using immunohistochemical methods, we describe the localization of the enzyme SAH hydrolase in rat kidney with a highly specific antibody raised in rabbits against purified SAH hydrolase from bovine kidney. This antibody crossreacts to almost the same extent with the SAH hydrolase from different species such as rat, pig, and human. Using light microscopy, SAH hydrolase was visualized by the biotin-streptavidin-alkaline phosphatase immunohistochemical procedure. SAH hydrolase immunostaining was observed in glomeruli and in the epithelium of the proximal and distal tubules. The collecting ducts of the cortex and medulla were homogeneously stained. By using double immunofluorescence staining and two-channel immunofluorescence confocal laser scanning microscopy, we differentiated the glomerular cells (endothelium, mesangium, podocytes) and found intensive staining of podocytes. Our results show that the enzyme SAH hydrolase is found ubiquitously in the rat kidney. The prominent staining of SAH hydrolase in the podocytes may reflect high rates of transmethylation. (J Histochem Cytochem 48:211-218, 2000)