The indirect role of biotin in the synthesis of the malic enzyme.

Although biotin is required for the synthesis of the inducible malic enzyme in Lactobacillus arabinosus 17-5 (I), partially purified preparations of the enzyme do not contain biotin (a), and avidin does not affect the activity of the enzyme (3). The formation of another enzyme, ornithine transcarbamylase, in iStreptococcus Zactis 8039 is also diminished by a biotin deficiency (4), but purified preparations of the enzyme similarly do not contain appreciable amounts of biotin (5). A recent study (6) has indicated that the function of biotin in the synthesis of ornithine transcarbamylase is concerned with the biogenesis of a 4 carbon unit which is essential for the synthesis of the enzyme. This 4 carbon unit can be synthesized by the organism through a reaction involving biotin or can be supplied exogenously by derivatives of aspartic acid such as glycylasparagine. In view of these results, it seemed worthwhile to reinvestigate the role of biotin in the synthesis of the malic enzyme. In the present investigation, it was found that asparagine and asparagine peptides, but not aspartic acid, replace the biotin requirement for malic enzyme synthesis in cells of L. arabinosus grown under biotinlimiting conditions. Aspartic acid not only does not replace the biotin requirement but also inhibits the stimulation of enzyme synthesis by biotin. The inability of the cells to utilize exogenous aspartic acid in place of biotin can be altered by certain changes in the growth medium or by preincubation of the cells.