Pyruvic acid labelled with isotopic

Our laboratory has studied the stereochemistry of methyl group formation in a number of a, 0 elimination reactions of amino acids catalyzed by pyridoxal phosphate enzymes. The reactions include the conversions of L-serine to pyruvate with tryptophan synthase 02 protein (78) and tryptophanase (79), of L-serine and l-tyrosine with tyrosine phenol-lyase (80), and l-cystine with S-alkylcysteine lyase (81). In the latter study, the stereospecific isotopically labeled L-cystines were obtained enzymatically by incubation of L-serines appropriately labeled in the 3-position with the enzyme O-acetyl serine sulfhy-drase (82). The serines tritiated in the 3-position were prepared enzymatically starting from [l-3H]glucose and [l-3H]mannose by a sequence of reactions of known stereochemistry (81). The cysteines were then incubated with 5-alkyl-cysteine lyase in 2H20 as outlined in Scheme 19. The pyruvate was trapped as lactate, which was oxidized with K2Cr202 to acetate for analysis. Similarly, Cheung and Walsh (71) examined the conversion of D-serine to pyruvate with... 

FIGURE 2. The pyruvate and the acetate pool are both needed for the formation of the introductory compound of plastidic Mev pathway. Isoprenoid (Car -H PQ) and fatty acid (16 0 -M8 1) formation in developing chloroplasts was stud j.ed in an isotopic dilution experiment. Chloroplasts (31 - 38 pg chi ml vol 1 ml) were supplied with the labeled precursor and increasing cunounts of jnlabeled substrate in the following combinations. Absolute values of C-incorporation j.nt(j> isoprenoids and fatty acids, in nmol C incorporated (mg chi) h -100, are indicated by brackets //. FIG. 2A 5 mM NaH CO (540 KBq)... 

Origin of the side chain of the aromatic amino acids and of prephenic acid from a three-carbon glycolytic intermediate was established by isotopic results as previously mentioned. Evidence of this is that the / -carbon of tyrorine is derived about equally and almost entirely from C-1 and 6 of glucose (217), and that the o-carbon was h hly labeled while the /3-carbon was unlabeled in tyrorane derived from pyruvate-a-C (219). Synthesis of prephenic acid presumably results from condensation of a pyruvate moiety with a cyclic intermediate beyond shikimic acid that has lost two hydroxyl groups. 

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