REGULATION OF POLY-BETA-HYDROXYBUTYRATE BIOSYNTHESIS BY NICOTINAMIDE NUCLEOTIDE IN ALCALIGENES-EUTROPHUS

Poly-beta-hydroxybutyrate biosynthesis was studied in Alcaligenes eutr ophus under various nutrient-limiting conditions. When the cells were cultivated in nitrogen-limited media, both the levels of NAD(P)H and t he ratios of NAD(P)H/NAD(P) were higher than those under nitrogen-suff icient conditions. The specific poly-beta-hydroxybutyrate production r ate was found to increase with the values of both NADH/NAD and NADPH/N ADP, indicating that poly-beta-hydroxybutyrate synthesis is directly r egulated by the ratios of nicotinamide nucleotides. The effects of nic otinamide nucleotides on poly-beta-hydroxybutyrate biosynthesis was in vestigated with regard to enzyme kinetics. Citrate synthase activity w as significantly inhibited by NADH and NADPH, indicating that poly-bet a-hydroxybutyrate accumulation could be enhanced by facilitating the m etabolic flux of acetyl-CoA to poly-beta-hydroxybutyrate synthetic pat hway. It was also found that cellular NADPH was a limiting substrate f or NADPH-linked reductase, controlling the overall biosynthetic activi ty of poly-beta-hydroxybutyrate in this strain.