Pyruvate dehydrogenase activity during ripening of hamlin oranges

Abstract

Pyruvate dehydrogenase (PD) was examined as a possible regulatory enzyme in the decline in aerobic respiration and increase in anaerobic metabolism in the ripening Hamlin orange. Oranges were harvested weekly over the growing season from October to February. Juice vesicles were excised and analysed for PD and the cofactors, NADH, NAD, ATP and ADP as well as for reduced and oxidized ubiquinone. PD levels increased slightly from October through February. The cofactor ratio of ATP/ADP increased slightly during that period, NADH/NAD increased more than 2-fold and ubiquinone became more reduced. Data suggest that PD could function as a regulatory enzyme in pyruvate metabolism and that either substrate levels of NAD dehydrogenases increased beyond the capacity of the respiratory pathway to adjust to a higher flux, or the oxidative function of the pathway declined over the season.