Escherichia coli possessing the dihydroxyacetone phosphate shunt utilize 5'-deoxynucleosides for growth

All organisms utilize S-adenosyl-l-methionine (SAM) as a key co-substrate for the methylation of biological molecules, the synthesis of polyamines, and radical SAM reactions. When these processes occur, 5'-deoxy-nucleosides are formed as byproducts such as S-adenosyl-l-homocysteine, 5'-methylthioadenosine (MTA), and 5'-deoxyadenosine (5dAdo). A prevalent pathway found in bacteria for the metabolism of MTA and 5dAdo is the dihydroxyacetone phosphate (DHAP) shunt, which converts these compounds into dihydroxyacetone phosphate and 2-methylthioacetaldehyde or acetaldehyde, respectively. Previous work in other organisms has shown that the DHAP shunt can enable methionine synthesis from MTA or serve as an MTA and 5dAdo detoxification pathway. Rather, the DHAP shunt in Escherichia coli ATCC 25922, when introduced into E. coli K-12, enables the use of 5dAdo and MTA as a carbon source for growth. When MTA is the substrate, the sulfur component is not significantly recycled back to methionine but rather accumulates as 2-methylthioethanol, which is slowly oxidized non-enzymatically under aerobic conditions. The DHAP shunt in ATCC 25922 is active under oxic and anoxic conditions. Growth using 5-deoxy-d-ribose was observed during aerobic respiration and anaerobic respiration with Trimethylamine N-oxide (TMAO), but not during fermentation or respiration with nitrate. This suggests the DHAP shunt may only be relevant for extraintestinal pathogenic E. coli lineages with the DHAP shunt that inhabit oxic or TMAO-rich extraintestinal environments. This reveals a heretofore overlooked role of the DHAP shunt in carbon and energy metabolism from ubiquitous SAM utilization byproducts and suggests a similar role may occur in other pathogenic and non-pathogenic bacteria with the DHAP shunt.

IMPORTANCE

The acquisition and utilization of organic compounds that serve as growth substrates are essential for Escherichia coli to grow and multiply. Ubiquitous enzymatic reactions involving S-adenosyl-l-methionine as a co-substrate by all organisms result in the formation of the 5'-deoxy-nucleoside byproducts, 5'-methylthioadenosine and 5'-deoxyadenosine. All E. coli possess a conserved nucleosidase that cleaves these 5'-deoxy-nucleosides into 5-deoxy-pentose sugars for adenine salvage. The DHAP shunt pathway is found in some extraintestinal pathogenic E. coli, but its function in E. coli possessing it has remained unknown. This study reveals that the DHAP shunt enables the utilization of 5'-deoxy-nucleosides and 5-deoxy-pentose sugars as growth substrates in E. coli strains with the pathway during aerobic respiration and anaerobic respiration with TMAO, but not fermentative growth. This provides an insight into the diversity of sugar compounds accessible by E. coli with the DHAP shunt and suggests that the DHAP shunt is primarily relevant in oxic or TMAO-rich extraintestinal environments.

Archaebacterial class I and class II aldolases from extreme halophiles

Both, class I (Schiff-base forming) and class II (metal requiring) fructose biphosphate aldolases were found to be distributed among halophilic archaebacteria. The aldolase activity from Halobacteriium halobium, H. salinarium, H. cutirubrum, H. mediterranei and H. volcanii exhibited properties of a bacterial class II aldolase as it was metal-dependent for activity and therefore inhibited by EDTA. In contrast, aldolase from H. saccharovorum, Halobacterium R-113, H. vallismortis and Halobacterium CH-1 formed a Schiff-base intermediate with the substrate and therefore resembled to eukaryotic class I type. The type of aldolase did not vary by changes in the growth medium.

The assay of methylglyoxal in biological systems by derivatization with 1,2-diamino-4,5-dimethoxybenzene

A procedure for the assay of methylglyoxal in biological systems is described, together with sample storage, sample processing procedures, and statistical evaluation. Specimen data are presented. Methylglyoxal was assayed by derivatization with 1,2-diamino-4,5-dimethoxybenzene and high-performance liquid chromatography (HPLC) of the resulting quinoxaline, 6,7-dimethoxy-2-methylquinoxaline, with spectrophotometric or fluorescence detection. Derivatization, solid-phase extraction, and HPLC were performed under acid conditions to prevent the spontaneous formation of methylglyoxal from glyceraldehyde 3-phosphate and dihydroxyacetone phosphate during the assay. The limits of detection in the biological matrix were 45 pmol (absorbance detection) and 10 pmol (fluorimetric detection), the recovery was 58%, and the intra- and interbatch coefficients of variance were 7.7 and 30.0%, respectively. The concentration of methylglyoxal in whole blood from normal healthy human individuals was (mean +/- SE, nM) 256 +/- 92 (n = 12) and that from diabetic patients was 479 +/- 49 (n = 55), showing a significant increase in diabetes mellitus (P < 0.01; Mann-Whitney U test). Sample processing under acidic conditions was essential to avoid interferences. Previous estimates of the concentration of methylglyoxal in biological samples require re-evaluation.

Metabolism of fructose-1,6-bisphosphate by mature boar spermatozoa

Mature epididymal boar spermatozoa converted glucose and fructose to carbon dioxide and lactate and maintained high concentrations of ATP. In the presence of (S)-alpha-chlorohydrin these processes were inhibited and there was an accumulation of fructose-1,6-bisphosphate and dihydroxyacetone phosphate. With fructose-1,6-bisphosphate as the substrate, the concentration of ATP was maintained, carbon dioxide was evolved and dihydroxyacetone phosphate accumulated. Cells pre-incubated with (S)-alpha-chlorohydrin did not maintain ATP levels, evolved less carbon dioxide and produced dihydroxyacetone phosphate. Assays of incubates in which fructose-1,6-bisphosphate was used as the substrate showed the presence of equilibrium quantities of fructose-6-phosphate and glucose-6-phosphate which were not detected when either fructose or glucose were used as substrates. [14C]Fructose and [14C]glucose were not produced from [14C]fructose-1,6-bisphosphate in spermatozoal incubates which had or had not been pre-incubated with (S)-alpha-chlorohydrin. Evidence is presented that a high concentration of fructose-1,6-bisphosphate leads to the formation of fructose-6-phosphate and glucose-6-phosphate but not of fructose and/or glucose.

Triosephosphate isomerase: removal of a putatively electrophilic histidine residue results in a subtle change in catalytic mechanism

An important active-site residue in the glycolytic enzyme triosephosphate isomerase is His-95, which appears to act as an electrophilic component in catalyzing the enolization of the substrates. With the techniques of site-directed mutagenesis, His-95 has been replaced by Gln in the isomerase from Saccharomyces cerevisiae. The mutant isomerase has been expressed in Escherichia coli strain DF502 and purified to homogeneity. The specific catalytic activity of the mutant enzyme is less than that of wild type by a factor of nearly 400. The mutant enzyme can be resolved from the wild-type isomerase on nondenaturing isoelectric focusing gels, and an isomerase activity stain shows that the observed catalytic activity indeed derives from the mutant protein. The inhibition constants for arsenate and for glycerol phosphate with the mutant enzyme are similar to those with the wild-type isomerase, but the substrate analogues 2-phosphoglycolate and phosphoglycolohydroxamate bind 8- and 35-fold, respectively, more weakly to the mutant isomerase. The mutant enzyme shows the same stereospecificity of proton transfer as the wild type. Tritium exchange experiments similar to those used to define the free energy profile for the wild-type yeast isomerase, together with a new method of analysis involving 14C and 3H doubly labeled substrates, have been used to investigate the energetics of the mutant enzyme catalyzed reaction. When the enzymatic reaction is conducted in tritiated solvent, the mutant isomerase does not catalyze any appreciable exchange between protons of the remaining substrate and those of the solvent either in the forward reaction direction (using dihydroxyacetone phosphate as substrate) or in the reverse direction (using glyceraldehyde phosphate as substrate). However, the specific radioactivity of the product glyceraldehyde phosphate formed in the forward reaction is 31% that of the solvent, while that of the product dihydroxyacetone phosphate formed in the reverse reaction is 24% that of the solvent. The deuterium kinetic isotope effects observed with the mutant isomerase using [1(R)-2H]dihydroxyacetone phosphate and [2-2H]glyceraldehyde 3-phosphate are 2.15 +/- 0.04 and 2.4 +/- 0.1, respectively. These results lead to the conclusion that substitution of Gln for His-95 so impairs the ability of the enzyme to stabilize the reaction intermediate that there is a change in the pathways of proton transfer mediated by the mutant enzyme. The data allow us more closely to define the role of His-95 in the reaction catalyzed by the wild-type enzyme, while forcing us to be alert to subtle changes in mechanistic pathways when mutant enzymes are generated.

Triosephosphate isomerase: energetics of the reaction catalyzed by the yeast enzyme expressed in Escherichia coli

Triosephosphate isomerase from bakers' yeast, expressed in Escherichia coli strain DF502(p12), has been purified to homogeneity. The kinetics of the reaction in each direction have been determined at pH 7.5 and 30 degrees C. Deuterium substitution at the C-2 position of substrate (R)-glyceraldehyde phosphate and at the 1-pro-R position of substrate dihydroxyacetone phosphate results in kinetic isotope effects on kcat of 1.6 and 3.4, respectively. The extent of transfer of tritium from [1(R)-3H]dihydroxyacetone phosphate to product (R)-glyceraldehyde phosphate during the catalyzed reaction is only 3% after 66% conversion to product, indicating that the enzymic base that mediates proton transfer is in rapid exchange with solvent protons. When the isomerase-catalyzed reaction is run in tritiated water in each direction, radioactivity is incorporated both into the remaining substrate and into the product. In the "exchange-conversion" experiment with dihydroxyacetone phosphate as substrate, the specific radioactivity of remaining dihydroxyacetone phosphate rises as a function of the extent of reaction with a slope of about 0.3, while the specific radioactivity of the products is 54% that of the solvent. In the reverse direction with (R)-glyceraldehyde phosphate as substrate, the specific radioactivity of the product formed is only 11% that of the solvent, while the radioactivity incorporated into the remaining substrate (R)-glyceraldehyde phosphate also rises as a function of the extent of reaction with a slope of 0.3. These results have been analyzed according to the protocol described earlier to yield the free energy profile of the reaction catalyzed by the yeast isomerase.(ABSTRACT TRUNCATED AT 250 WORDS)

Alkyldihydroxyacetonephosphate synthase mechanism: 18O studies of fatty acid release from acyldihydroxyacetone phosphate

Alkyldihydroxyacetonephosphate synthase (alkyl-DHAP synthase) catalyzes the exchange of the ester-linked fatty acid of 1-O-acyldihydroxyacetone phosphate (1-O-acyl-DHAP) for a fatty alcohol that is attached in an ether linkage to form 1-O-alkyldihydroxyacetone phosphate (1-O-alkyl-DHAP). In our continuing investigation of the mechanism of this enzyme, we have examined the fatty acid released during the reaction. In contrast to the reports of others using whole microsomes, we found that the cleavage of fatty acid by purified preparations of alkyl-DHAP synthase was dependent on the presence of the cosubstrate, fatty alcohol. Furthermore, the amount of fatty acid produced was equivalent to the alkyl-DHAP formed. Our previously proposed detailed mechanism for alkyl-DHAP synthase predicted that the fatty acid should retain both of the carboxyl ester oxygens upon cleavage. Reactions carried out with palmitoyl-[18O]DHAP as substrate yielded [18O]palmitic acid as the product in agreement with this scheme.

Characterization of methylglyoxal synthase in Saccharomyces cerevisiae

Methylglyoxal synthase in Saccharomyces cerevisiae was purified approximately 300 folds from cell extracts with 20% of activity yield. During purification procedures, polymorphic behaviours of the enzyme were observed. The purified enzyme was homogeneous on polyacrylamide gel electrophoresis and consisted of a single polypeptide chain of Mr = 26,000. The enzyme was most active at pH 9.5-10.5 and strictly specific to dihydroxyacetone phosphate with Km = 3 mM. Phosphoenolpyruvate, glyceraldehyde-3-phosphate, orthophosphate and thiol compounds were potent inhibitors of the enzyme.

Absence of alpha-glycerophosphate dehydrogenase in axenically grown Entamoeba histolytica

We conclude that there is no evidence for the presence of alpha-glycerophosphate dehydrogenase among the cytoplasmic enzymes of Entamoeba histolytica, and that a contrary finding was probably caused by the action of a different amebal enzyme on unrecognized contamination in the lithium salt of the substrate used in that investigation.

The mechanism of ether bond formation in O-alkyl lipid synthesis

O-Alkyl dihydroxyacetone phosphate is formed enzymatically from acyl dihydroxyacetone phosphate and a long chain fatty alcohol. This reaction is accompanied by stereospecific exchange of the pro-R hydrogen of carbon 1 (carbon 1 of all compounds corresponds to carbon 1 of sn-glycerol) of the dihydroxyacetone phosphate moiety with retention of configuration. In the present investigation, data are provided to show that the initial loss of hydrogen from carbon 1 of acyl dihydroxyacetone phosphate does not depend on the presence of the fatty alcohol. In addition, the occurrence of a Schiff base between enzyme and acyl dihydroxyacetone phosphate, comparable to the fructose-1,6-diphosphate aldolase reaction, could not be demonstrated. It is concluded that the formation of 1-O-alkyl dihydroxyacetone phosphate via the formation of intermediate 1-O-acyl endiol and 1-O-alkyl endiol is a likely mechanism.

Direct observation of substrate distortion by triosephosphate isomerase using Fourier transform infrared spectroscopy

The infrared spectrum of dihydroxyacetone phosphate bound to triosephosphate isomerase has been measured. There are two carbonyl bands corresponding to the bound substrate, with an intensity ratio of about 3:1. Relative to the carbonyl absorption of dihydroxyacetone phosphate in free solution, the major band is shifted by 19 cm-1 to 1713 cm-1, providing direct evidence of enzyme-induced distortion of the substrate. This strain is probably attributable to an enzymic electrophile that polarizes the carbonyl group of the substrate and thereby promotes catalysis.

Partial purification, substrate specificity and regulation of alpha-L-glycerolphosphate dehydrogenase from Saccharomyces carlsbergensis

alpha-L-Glycerolphosphate dehydrogenase (sn-glycerol-3-phosphate:NAD+ 2-oxidoreductase, EC 1.1.1.8) from Saccharomyces carlsbergensis was purified 400-fold. The enzyme preparation is free of interfering activities, such as glyceraldehyde phosphate dehydrogenase, alcohol dehydrogenase, triose phosphate isomerase and glycerolphosphatase. At pH 7.0 it is specific for NADH (Km = 0.027 mM with 0.8 mM dihydroxyacetone phosphate) and dihydroxyacetone phosphate (Km = 0.2 mM with 0.2 mM NADH). Between pH 5.0 and 6.0 the enzyme functions with NADPH, but only at 7% of the rate with NADH. Various anions (I- greater than SO42- greater than Br- greater than Cl-) act as inhibitors competing with the substrate dihydroxyacetone phosphate. Inorganic phosphate (Ki = 0.1 mM), pyrophosphate and arsenate are strong inhibitors. The nucleotides ATP and ADP are also inhibitory, but their action seems to be of the same type as the general anion competition (Ki = 0.73 mM for ATP). The results are consistent with the notion that the enzyme may regulate the redox potential of the NAD+/NADH couple during fermentation.

Subcellular localization of acyl coenzyme A: dihydroxyacetone phosphate acyltransferase in rat liver peroxisomes (microbodies)

Upon differential centrifugation of rat liver homogenate, the enzyme acyl-CoA:dihydroxyacetone-phosphate acyltransferase (EC 2.3.1.42) was found to be localized in the light mitochondrial (L) fraction which is enriched with lysosomes and peroxisomes. Peroxisomes were separated from lysosomes in a density gradient centrifugation using rats which were injected with Triton WR 1339. By comparing the enzyme distribution with the distribution of different marker enzymes, it was concluded that dihydroxyacetone phosphate acyltransferase is primarily localized in rat liver peroxisomes (microbodies). Similarly, the enzyme acyl dihydroxyacetone-phosphate:NADPH oxidoreductase (EC 1.1.1.101) was shown to be enriched in the peroxisomal fraction, although a portion of this reductase is also present in the microsomal fraction.

Fructose-1,6-bisphosphate aldolase from rabbit liver. Reaction mechanism and physiological function

Liver and muscle aldolase display similar reaction mechanisms. Both the enzymes, by reacting with dihydroxyacetone phosphate, form an acid-labile intermediate which is in rapid equilibrium with an eneamine intermediate. Differences are found in the equilibrium concentration of the acid-labile intermediate, which represents approximately 25% of the total intermediates in the liver (this paper) and 60% in the muscle enzyme [E. Grazi and G. Trombetta, Biochem. J. 175, 361 (1978)] and in the rate of formation of the eneamine intermediate which is much slower in the liver enzyme. Furthermore, with liver aldolase, the rate by which the C-3H bond of dihydroxyacetone phosphate is cleaved is increased by 60 times in the presence of glyceraldehyde 3-phosphate. This, mechanistically, indicates that glyceraldehyde 3-phosphate is bound to the enzyme before the formation of the eneamine from dihydroxyacetone phosphate, and, physiologically, that in liever aldolase the gluconeogenetic activity is favoured over the glycolytic activity.

Relative distribution in parenchymal and non-parenchymal cells, effects of N-ethylmaleimide, palmitoyl-coenzyme A concentration, starvation, adrenalectomy and anti-insulin serum treatment

1. GPAT (glycerol phosphate acyltransferase) and DHAPAT (dihydroxyacetone phosphate acyltransferase) activities were measured both in subcellular fractions prepared from fed rat liver and in whole homogenates prepared from freeze-stopped pieces of liver. 2. GPAT activity in mitochondria differed from the microsomal activity in that it was insensitive to N-ethylmaleimide, had a higher affinity towards the palmitoyl-CoA substrate and showed a different response to changes in hormonal and dietary status. 3. Starvation (48 h) significantly decreased mitochondrial GPAT activity. The ratio of mitochondrial to microsomal activities was also significantly decreased. The microsomal activity was unaffected by starvation, except after adrenalectomy, when it was significantly decreased. Mitochondrial GPAT activity was decreased by adrenalectomy in both fed and starved animals. 4. Acute administration of anti-insulin serum significantly decreased mitochondrial GPAT activity after 60 min without affecting the microsomal activity. 5. A new assay is described for DHAPAT. The subcellular distribution of this enzyme differed from that of GPAT. The highest specific activity of DHAPAT was found in a 23 000 gav. pellet obtained by centrifugation of a post-mitochondrial supernatant. This fraction also contained the highest specific activity of the peroxisomal marker uricase. DHAPAT activity in mitochondrial fractions or in the 23 000 gav. pellet was stimulated by N-ethylmaleimide, whereas that in microsomal fractions was slightly inhibited by this reagent. The GPAT and DHAPAT activities in mitochondrial fractions had a considerably higher affinity for the palmitoyl-CoA substrate. 6. Total liver DHAPAT activity was significantly decreased by starvation (48 h), but was unaffected by administration of anti-insulin serum. 7. The specific activities of GPAT and DHAPAT were lower in non-parenchymal cells compared with parenchymal cells, but the GPAT/DHAPAT ratio was 5--6-fold higher in the parenchymal cells.

Stereochemical specificity of the biosynthesis of the alkyl ether bond in alkyl ether lipids

The stereochemical course of the formation of the alkyl ether bond in alkyl ether lipids was investigated through the synthesis of stereospecifically labeled acyl R- or S-[1-3H]dihydroxyacetone 3-phosphate (DHAP) starting from L-glyceraldehyde. It was demonstrated directly that the formation of the alkyl ether bond results in the stereospecific exchange of the pro-R C-1 hydrogen of DHAP with a proton of water. The configuration of the hydrogen that is retained on C-1 after formation of the alkyl ether bond was also investigated. The alkyl ether lipid was degraded, and the DHAP backbone isolated as glycerol, converted to DHAP via glycerol 3-phosphate and treated with either aldolase or triose phosphate isomerase. The results demonstrated that the retained hydrogen on C-1, which was pro-S in the starting substrate, was pro-S in the product alkyl ether.

Enzyme-substrate and enzyme-inhibitor complexes of triose phosphate isomerase studied by 31P nuclear magnetic resonance

The complex formed between the enzyme triose phosphate isomerase (EC 5.3.1.1.), from rabbit and chicken muscle, and its substrate dihydroxyacetone phosphate was studied by 31P n.m.r. Two other enzyme-ligant complexes examined were those formed by glycerol 3-phosphate (a substrate analogue) and by 2-phosphoglycollate (potential transition-state analogue). Separate resonances were observed in the 31P n.m.r. spectrum for free and bound 2-phosphoglycollate, and this sets an upper limit to the rate constant for dissociation of the enzyme-inhibitor complex; the linewidth of the resonance assigned to the bound inhibitor provided further kinetic information. The position of this resonance did not vary with pH but remained close to that of the fully ionized form of the free 2-phosphoglycollate. It is the fully ionized form of this ligand that binds to the enzyme. The proton uptake that accompanies binding shows protonation of a group on the enzyme. On the basis of chemical and crystallographic information [Hartman (1971) Biochemistry 10, 146--154; Miller & Waley (1971) Biochem. J. 123, 163--170; De la Mare, Coulson, Knowles, Priddle & Offord )1972) Biochem. J. 129, 321--331; Phillips, Rivers, Sternberg, Thornton & Wilson (1977) Biochem. Soc. Trans. 5, 642--647] this group is believed to be glutamate-165. On the other hand, the position of the resonance of D-glycerol 3 phosphate (sn-glycerol 1-phosphate) in the enzyme-ligand complex changes with pH, and both monoanion and dianon of the ligand bind, although dianion binds better. The substrate, dihydroxyacetone phosphate, behaves essentially like glycerol 3-phosphate. The experiments with dihydroxy-acetone phosphate and triose phosphate isomerase have to be carried out at 1 degree C because at 37 degrees C there is conversion into methyl glyoxal and orthophosphate. The mechanismof the enzymic reaction and the reasons for rate-enhancement are considered, and aspects of the pH-dependence are discussed in an Appendix.

Glycerolipid biosynthesis in Saccharomyces cerevisiae: sn-glycerol-3-phosphate and dihydroxyacetone phosphate acyltransferase activities

Yeast acyl-coenzyme A:dihydroxyacetone-phosphate O-acyltransferase (DHAP acyltransferase; EC 2.3.1.42) was investigated to (i) determine whether its activity and that of acyl-coenzyme A:sn-glycerol-3-phosphate O-acyltransferase (glycerol-P acyltransferase; EC 2.3.1.15) represent dual catalytic functions of a single membranous enzyme, (ii) estimate the relative contributions of the glycerol-P and DHAP pathways for yeast glycerolipid synthesis, and (iii) evaluate the suitability of yeast for future genetic investigations of the eucaryotic glycerol-P and DHAP acyltransferase activities. The membranous DHAP acyltransferase activity showed an apparent Km of 0.79 mM for DHAP, with a Vmax of 5.3 nmol/min per mg, whereas the glycerol-P acyltransferase activity showed an apparent Km of 0.05 mM for glycerol-P, with a Vmax of 3.4 nmol/min per mg. Glycerol-P was a competitive inhibitor (Ki, 0.07 mM) of the DHAP acyltransferase activity, and DHAP was a competitive inhibitor (Ki, 0.91 mM) of the glycerol-P acyltransferase activity. The two acyltransferase activities exhibited marked similarities in their pH dependence, acyl-coenzyme A chain length preference and substrate concentration dependencies, thermolability, and patterns of inactivation by N-ethylmaleimide, trypsin, and detergents. Thus, the data strongly suggest that yeast glycerol-P and DHAP acyltransferase activities represent dual catalytic functions of a single membrane-bound enzyme. Furthermore, since no acyl-DHAP oxidoreductase activity could be detected in yeast membranes, the DHAP pathway for glycerolipid synthesis may not operate in yeast.

Absolute configuration of tritiated O-alkylglycerol synthesized enzymatically from (1,3-3H2, 1,3-14C2)dihydroxyacetone phosphate

O-Alkyldihydroxyacetone phosphate is synthesized enzymatically from hexadecanol and acyldihydroxyacetone phosphate. In this process there is a hydrogen exchange in which the pro-R hydrogen of C-1 of the sn-glycerol moiety is lost. This hydrogen is replaced by a hydrogen from the medium. In order to obtain additional information on the mechanism of ether bond formation, it would be of interest to know whether or not the hydrogen exchange results in a change of configuration in the product, O-alkyldihydroxyacetone phosphate. By using O-alkylglycerol prepared both chemically and enzymatically from isomerase-treated [1,3-3H2, 1,3-14C2] dihydroxyacetone phosphate and an O-alkylglycerol cleavage enzyme system, it was shown that the hydrogen exchange occurs with retention of configuration of the substituents of C-1 of the sn-glycerol moiety.

Properties of dihydroxyacetone phosphate acyltransferase in the harderian gland

We have used a microsomal preparation from the pink portion of the rabbit harderian gland to study the properties of dihydroxyacetone-P acyltransferase. The enzymatic activity in the microsomes is latent and is stimulated approximately 20-fold by the addition of detergent to the assay system. Both deoxycholate and cholate stimulated the enzyme at concentrations considerably below the critical micellar concentration of these detergents. The acyltransferase was not solubilized by treatment with these detergent concentrations. In addition, we have shown that the acyltransferase is sensitive to proteolytic digestion in the presence of deoxycholate concentrations that render the microsomal vesicles permeable to the protease, but is insensitive to the protease treatment in the absence of detergent. Collectively, these data indicate that the active site of dihydroxyacetone-P acyltransferase is exposed to the lumenal surface of the microsomal vesicles. This enzyme is distinct from the glycerol-P acyltransferase in that the dihydroxyacetone-P acyltransferase is sensitive to protease digestion only in the presence of deoxycholate, is not inhibited by sulfhydryl reagents, and is not competitively inhibited by sn-glycerol-3-P.

Nonenzymatic glycosylation of hemoglobin

The incubation of dialyzed hemoglobin A with a number of phosphorylated glycolytic intermediates leads to the formation of covalent hemoglobin adducts that co-chromatograph with hemoglobin AIb. Phosphorylated hexoses (glucose-6-P, fructose-6-P, fructose-1,6-P2) and trioses (glyceraldelyde-3-P, dihydroxyacetone-P) containing a free aldehyde or ketone can glycosylate hemoglobin A nonenzymatically. From 7 to 12% of the hemoglobin can be modified after a 72-h incubation of an equimolar mixture of hemoglobin A and the phosphorylated intermediate. No significant formation of adduct was seen with a sugar alone (glucose, fructose) or glycolytic intermediate which had a blocked aldehyde (glucose-1-P, glucose-1,6-P2, UDP-glucose). The addition of an equimolar amount of 2,3-diphosphoglycerate reduced adduct formation. Evidently, the phosphate is needed to orient and stabilize the intermediate in the bisphosphoglycerate pocket of hemoglobin so that the addition reaction can proceed. All of the hemoglobin A adducts were indistinguishable form hemoglobin AIb by ion exchange chromatography and isoelectric focusing. The hemoglobin A-glucose-6-P adduct and hemoglobin AIb had a NaB3H4-reducible linkage in the beta chain. The concentration of hemoglobin AIb is elevated in patients with diabetes mellitus. This presumably reflects the increased concentrations of glycolytic intermediates (glucose-6-P, fructose-6-P, fructose-1,6-P2, dihydroxyacetone-P) which were found to be significantly elevated in the red cells of diabetic patients as compared with normal controls.

Fructose 1,6-diphosphate aldolase from rabbit muscle. Effect of pH on the rate of formation and on the equilibrium concentration of the carbanion intermediate

The rate of oxidation of ferricyanide of the aldolase-dihydroxyacetone phosphate complex was measured under different conditions. The following conclusions are drawn. 1. In the cleavage of fructose diphosphate, catalysed by native aldolase, the steady-state concentration of the enzyme-dihydroxyacetone phosphate carbanion intermediate represents less than 6% of the total enzyme-substrate intermediates. 2. Fructose diphosphate and dihydroxyacetone phosphate compete for the four catalytic sites on aldolase, the binding of fructose diphosphate being about twice as tight. 3. The equilibrium concentration of the carbanion intermediate formed by reaction of carboxypeptidase-treated aldolase with dihydroxyacetone phosphate is independent of pH between 5.0 and 9.0. The rates of fromation of the carbanion intermediate and of the reverse reaction are, however, concomitantly increased by increasing pH between 5.0 and 6.5.

Cycloalkanones V: synthesis, distribution, and effects on triglyceride metabolism

The 14-C-labeled 2,8-dibenzylcyclooctanone was synthesized to study its absorption, distribution, and excretion in rats. Maximum drug absorption from the GI tract occurred between 12 and 14 hr after administration. The major organs possessed maximum amounts of the drug in 1 hr, with the liver concentrating the most with 6.56% 14-C and the muscle mass reaching a maximum of 41% 14-C after 14 hr. The drug remained in the GI tract over the first 6 hr and was associated with the lipid and glycogen fractions. Eighty-seven percent was eliminated in the feces after 72 hr. 2,8-Dibenzylcyclooctanone caused a significant reduction in vitro of dihydroxyacetone phosphatase acyltransferase and sn-glycerol-3-phosphate acyltransferase, which is the proposed mechanism for the observed in vivo reduction of hepatic, intestinal, and serum triglycerides and total glycerolipids. In vivo administration of the drug resulted in a depression of liver acid phosphatidyl phosphatase, acid phosphatase and lipase, and adipose lipase. The drug increased the rates of excretion of exogenous cholesterol, palmitic acid, and progesterone.

A study of the kinetics and mechanism of rabbit muscle L-glycerol 3-phosphate dehydrogenase

1. The kinetics of oxidation of l-glycerol 3-phosphate by NAD(+) and of reduction of dihydroxyacetone phosphate by NADH catalysed by rabbit muscle glycerol 3-phosphate dehydrogenase were studied over the range pH6-9. 2. The enzyme was found to catalyse the oxidation of glyoxylate by NAD(+) at pH8.0 and the kinetics of this reaction were also studied. 3. The results are consistent with a compulsory mechanism of catalysis for glycerol 3-phosphate oxidation and dihydroxyacetone phosphate reduction in the intermediate regions of pH, but modifications to the basic mechanism are required to fully explain results at the extremes of the pH range, with these substrates and for glyoxylate oxidation at pH8.0.