Acetolactate and Acetoin Synthesis in Ripening Peas

The role of acetolactate as a precursor in valine biosynthesis was first indicated by the isotopic studies of Strassman et al. (18) and was subsequently supported by work on valine-less mutants of Escherichia coli (21, 22) and Neurospora, crassa (24). The direct conversion of acetolactate to a number of valine precursors, and to valine itself, has since been demonstrated in cell-free extracts of a number of microorganisms (11, 15, 19, 25). A similar conversion of acetolactate to valine by crude extracts of spinach has been reported recently (27) and the enzymes responsible for this conversion demonstrated in cell-free extracts of Phaseolus radiatus (16). The formation of acetolactate from pyruvate has been observed in cell-free extracts of microorganisms (3, 5, 11, 14, 21) and pyruvate oxidase preparations from pigeon breast muscle (6). Such a synthesis has also been reported briefly by Satyanarayana and Radhakrishnan (16) for extracts of Phaseolus seedlings. The present communication reports on the properties of a partially purified preparation of acetolactate-forming enzymes from ripening pea seeds. The relationship between the enzymes responsible for acetoin and acetolactate synthesis is also considered.