Effect of Thiamin-induced Vitamin B6 Deficiency on NAD- and NADP-linked Glutamate Dehydrogenases in Yeast

Abstract

Thiamin-grown cells of Saccharomyces carlsbergensis exhibited decreased activity of NADlinked glutamate dehydrogenase (NAD-GDH) and, in contrast, increased activity of NADPlinked glutamate dehydrogenase (NADP-GDH), as compared with control cells. Normal levels of glutamate dehydrogenase (GDH) activities were restored by adding pyridoxine. 6-Aminolaevulinate also prevented the thiamin-induced decrease in NAD-GDH activity, but had no effect on the increase in NADP-GDH activity. NAD-GDH activity was decreased similarly under anaerobic conditions, but NADP-GDH activity was little affected. High concentrations of glucose brought about a decrease in NAD-GDH activity and an increase in NADP-GDH activity, as observed in the thiamin-grown cells. When glycerol was used as carbon source in place of glucose, the thiamin effect on GDH activities was not marked. These results suggest that GDH enzymes are independently controlled by thiamin. NAD-GDH activity is decreased mainly through the thiamin-induced respiratory deficiency, and the increase in NADP-GDH activity is due to the thiamin-enhanced glucose effect. Amino acid metabolism seemed not to be involved in the thiamin effect on GDH activities since the intracellular pools of NH,+ and glutamate were not altered by thiamin. The activities of glutamateoxaloacetate transaminase and glutamate-pyruvate transaminase were not influenced greatly by thiamin, unlike the activity of b-aminolaevulinate synthase.