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PubMed1983

Separate physiological roles and subcellular compartments for two tetrapyrrole biosynthetic pathways in Euglena gracilis.

delta-Aminolevulinic acid (ALA), the first committed precursor to the tetrapyrrole components of hemes and chlorophylls, is synthesized by two different routes in the photosynthetic phytoflagellate Euglena gracilis: directly from glutamate, mediated by a 5-carbon pathway, and via condensation of glycine and succinyl-CoA, catalyzed by the enzyme ALA synthase. The physiological roles of the two pathways were determined by administration of specifically 14C-labeled ALA precursors to cultures growing under different physiological conditions. Relative activities of the ALA synthase and 5-carbon pathways were monitored by incorporation of radioactivity from [2-14C] glycine and [1-14C]glutamate into highly purified protoheme, heme a and chlorophyll a derivatives. Wild type cells grown photoautotrophically or photoheterotrophically synthesized chlorophyll and incorporated radioactivity from [1-14C]glutamate into the tetrapyrrole nucleus of the pigment. [2-14C]Glycine was incorporated primarily into the nontetrapyrrole-derived portions of chlorophyll. In the same cultures both [2-14C]glycine and [1-14C]glutamate were efficiently incorporated into protoheme, while only [2-14C] glycine was incorporated into heme a. In dark-grown wild type or light-grown aplastidic cells, no chlorophyll was formed, and both protoheme and heme a were labeled exclusively from [2-14C]glycine. These results indicate: (a) ALA synthase and the 5-carbon pathway operate simultaneously in growing green cells; (b) the 5-carbon pathway provides ALA for chloroplast protoheme and chlorophyll, and is associated with chloroplast development; (c) ALA synthase provides ALA only for nonplastid heme biosynthesis; and (d) the two ALA pathways are separately compartmentalized along with complete sets of enzymes for subsequent tetrapyrrole synthesis from each ALA pool. The protoheme that was synthesized from [1-14C] glutamate had a higher specific radioactivity than chlorophyll synthesized from the same precursor. This result together with calculated specific radioactivities of the products synthesized during the incubation period, suggest that both protoheme and heme a undergo metabolic turnover.

5-Aminolevulinate Synthetase↗
PubMed1984

Interaction of aspartate and aspartate-derived antimetabolites with the enzymes of the threonine biosynthetic pathway of Escherichia coli.

The five enzymes responsible for the conversion of L-aspartate to L-threonine in Escherichia coli were purified to homogeneity and subsequently reconstituted in vitro in ratios approximating those found in vivo. 31P NMR was used to conveniently monitor the rates of consumption of the substrates ATP and NADPH, the accumulation of the intermediates beta-aspartyl phosphate and homoserine phosphate, and the formation of the products ADP, NADP+, and Pi in a single experiment. By this method, the flux of aspartic acid through the enzymes of the pathway was monitored in the absence and in the presence of several alternative substrates and inhibitors. Several known antimetabolites were found to be alternative substrates that ultimately became inhibitors of pathway flux. L-threo-3-Hydroxyaspartic acid was converted to 3-hydroxyhomoserine phosphate by the first four enzymes of the pathway. The antimetabolite L-threo-3-hydroxyhomoserine was found to bind to and inhibit aspartokinase-homoserine dehydrogenase I in a cooperative fashion (I 0.5 = 3 mM, nH = 2.5), similar to the action of the allosteric end product inhibitor L-threonine (I 0.5 = 0.36 mM, nH = 2.4). In the presence of the remaining enzymes of the pathway, however, L-threo-3-hydroxyhomoserine was phosphorylated to the apparent ultimate antimetabolite L-threo-3-hydroxyhomoserine phosphate that was a potent inhibitor of threonine synthase and consequently of L-threonine biosynthesis. When aspartic acid alone was examined as a substrate of the enzymes of the pathway, no accumulation of the beta-aspartyl phosphate and homoserine phosphate intermediates was observed. However, in the presence of either 5 mM L-threo-3-hydroxyhomoserine or 5 mM L-threo-3-hydroxyhomoserine phosphate, homoserine phosphate was found to accumulate. In contrast to the homoserine phosphate and 3-hydroxyhomoserine phosphate intermediates, both of which were very stable, the acylphosphate intermediates beta-aspartyl phosphate and beta-3-hydroxyaspartyl phosphate were highly susceptible to hydrolysis, with first-order rate constants of 4.6 X 10(-3) min-1 and 4.5 X 10(-2) min-1 (pH 7.8, 25 degrees C), respectively.

Aspartate Kinase↗
PubMed1983

The biosynthetic pathway of renin in mouse submandibular gland.

The present study set out to demonstrate the biosynthesis of a prorenin and its processing in the cell, using the richest known natural source of renin, the mouse submandibular gland. Cell-free translation of total poly(A+) RNA or mRNA selected using a renin cDNA yielded Mr = 45,000 preprorenin which in the presence of dog pancreatic microsomal membranes, was converted to Mr = 43,000 prorenin. The latter was seen during in vitro labeling of tissue with [35S]methionine. Prorenin was synthesized first and converted rapidly to Mr = 38,000 single chain renin. Renin was then hydrolyzed slowly to give two chains of Mr = 33,000 and 5,000 held together by disulfide bonds. The Mr = 38,000 and 33,000 species had similar peptide maps. Western blotting of fractions from a pepstatin affinity column identified the separation of prorenin from renin. The results suggested that both single and two-chain renin have an exposed active site. Testosterone stimulated synthesis of prorenin during in vitro labeling of female tissue. Thus, mouse renin is synthesized as a preprorenin (Mr = 45,000) which is converted to a prorenin (Mr = 43,000) and then to renin (Mr = 38,000) by rapid processing within the cell, after which renin is cut slowly to give a two-chain form.

Animals↗
PubMed1983

Structural studies of the sugar chains of hen ovomucoid. Evidence indicating that they are formed mainly by the alternate biosynthetic pathway of asparagine-linked sugar chains.

The carbohydrate moieties of hen ovomucoid were released as oligosaccharides by hydrazinolysis. The neutral oligosaccharide fraction which comprised about 85% of the total sugar was fractionated into eight oligosaccharide fractions by Bio-Gel P-4 column chromatography. Occurrence of novel penta-antennary oligosaccharides in the larger three fractions was reported in the preceding paper (Yamashita, K., Kamerling, J.P., and Kobata, A. (1982) J. Biol. Chem. 257, 12809-12814). Structural studies of the remaining smaller oligosaccharides indicated that they all have Man alpha 1 leads to 6(Man alpha 1 leads to 3)Man beta 1 leads to 4GlcNAc beta 1 leads to 4GlcNAc as their common core. The alpha-mannosyl residues occur either free or as one of the following five groups: GlcNAc beta 1 leads to 2Man, GlcNAc beta 1 leads to 4Man, GlcNAc beta 1 leads to 4(GlcNAc beta 1 leads to 2)Man, GlcNAc beta 1 leads to 6(GlcNAc beta 1 leads to 2)Man, and GlcNAc beta 1 leads to 6(GlcNAc beta 1 leads to 4)(GlcNAc beta 1 leads to 2) Man. In most oligosaccharides, a beta-N-acetylglucosamine residue is linked at the C-4 position of the beta-mannosyl residue of the core. The structural characteristic of the sugar chains of hen ovomucoid indicated that they are not formed by the ordinary processing pathway of the asparagine-linked sugar chains.

Acetylglucosamine↗
PubMed1980

Evaluation of biosynthetic pathways of phosphatidyl choline in liver of rats given phenobarbital: a study on molecular species of phosphatidyl choline.

Administration of phenobarbital to female rats significantly increased the levels of liver monoenoic, dienoic and tetraenoic PC and in plasma it raised phosphatidyl choline and its monoenoic and dienoic species and decreased triglyceride levels. Significant reductions in the incorporation of NaH2 32PO4 into monoenoic and dienoic PC were observed in phenobarbital treated rats. Phenobarbital administration significantly increased the incorporation of 14CH3-methionine into tetraenoic and hexaenoic PC of liver. In plasma the incorporation of NaH2 32PO4 into monoenoic and dienoic PC in phenobarbital treated rats was not affected but that of 14CH3-methionine incorporation into tetraenoic and hexaenoic PC was suppressed as compared to the control. Phenobarbital enhanced PC synthesis Via N-methylation of PE and depressed via CDP-choline pathway.

Animals↗