Biosynthesis of Valine and Isoleucine
نویسنده
چکیده
The isotope studies of Strassman et al. (3, 4) first implicated cu-acetolactate and cu-aceto-cY-hydroxybutyrate as biosynthetic precursors of valine and isoleucine, respectively. The observed accumulation of acetolactate by Escherichia coli (5, 6) and of the decarboxylation products of acetolactate and acetohydroxybutyrate by Neurospora crassa (7) when mutants of these organisms are grown with limiting amounts of valine and isoleucine is consistent with this role. More recently, the direct conversion of acetolactate to ol-ketoisovaleric acid in crude extracts of yeast (8, 9) and to valine in cell-free extracts of N. crassa (1) has been demonstrated. The formation of acetolactate from pyruvate in biological systems was first postulated by Krampitz (10) and has subsequently been demonstrated in crude cell-free extracts of Aerobacter aerogenes (11, 12), in E. coli (5, 6, 9, 13), and (contrary to earlier reports) in yeast (9). In contrast, the formation of cY-aceto-cuhydroxybutyrate by cell-free enzymes has not been demonstrated. We report here formation of the latter product by cellfree preparations from N. crussu, together with a comparison of the properties of the partially purified, acetolactate-forming enzymes from E. coli and N. crassu. A preliminary account of some of this work has appeared elsewhere (2).
منابع مشابه
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