STARVATION REDUCES PYRUVATE-DEHYDROGENASE PHOSPHATE PHOSPHATASE-ACTIVITY IN RAT-KIDNEY

Pyruvate dehydrogenase complex (PDC) from rat kidney or pig heart prev iously inactivated by phosphorylation (PDHP) was activated in vitro by PDHP phosphatase from kidneys of starved or fed rats. Starvation for 48 h of the rats from which the PDC was prepared led to a decrease in the rate of activation of PDC at early time periods (< 2 min), particu larly at submaximal concentrations of Mg2+. Using intact permeable kid ney mitochondria incubated for 15 sec, it was found that starvation of rats more than doubled the Mg2+ concentration at which the half maxim al increment of PDC activity (PDCa) was observed. Reduction of PDHP ph osphatase activity due to starvation was also apparent when phosphatas e was separated from PDC and recombined with PDC from the same or diff erent animals. Intraperitoneal injection of insulin and glucose 1 h be fore sacrifice of starved rats prevented the reduction of PDHP phospha tase activity whether or not protein synthesis was inhibited. The effe ct of insulin in restoration of PDHP phosphatase activity of starved r ats was not mimicked by 5-methylpyrazole 3-carboxylic acid, an inhibit or of lipolysis. When renal PDHP phosphatase was incubated with pig he art PDC in the presence of 10 mM Mg2+ and 0.1 mM Ca2+ the increment in PDCa, in 1 min was 30% of fully activated PDC activity (PDCt) observe d after 15 min. Removal of divalent cations did not affect the increme nt in 1 min but prevented further increments. Conversely okadaic acid diminished 1 min increment but did not disturb PDCt. It is suggested t hat the different behaviour of renal PDC from fed and starved animals may partly be due to different divalent cation independent PDHP phosph atase activity.