Inactivation and reactivation of rat liver 3-hydroxy-3-methylglutaryl-CoA-reductase phosphatases: effect of phosphate, pyrophosphate and divalent cations.

Abstract

Incubation of four purified rat liver HMG-CoA-reductase phosphatases (Gil, G., Sitges, M. and Hegardt, F.G. (1981) Biochim. Biophys. Acta 663, 211-221) with Mn2 or Mg2 caused a concentration-dependent activation of enzyme activities. The maximum effect for Mn2 was at 5 mM for all phosphatases. Fe2 caused inactivation only in reductase phosphatases IIa and IIb. Ca2 10 mM showed a slight effect of inactivation. Phosphate, pyrophosphate and adenine nucleotides inhibited the four reductase phosphatases, this process being concentration-dependent. cAMP did not inhibit the four phosphatases at all in the range of 0.01-8 mM. Preincubation of reductase phosphatases with PPi and subsequent dilution did not diminish the inactivation effect, showing that this ion inhibits the enzyme prior to the binding to the substrate. Phosphorylated sugars, but not free sugar, inactivated the four reductase phosphatases. PPi-inactivated enzymes were reactivated by Mg2 or Mn2, this process being time-dependent. The four phosphatases had different patterns of reactivation. Phosphatases Ib and IIb (low-molecular mass forms) were shown to be different enzymes as judged by: their divergent behaviour when inhibited with Fe2; their PPi response; kinetics of reactivation by Mg2 or Mn2 or PPi-inactivated enzymes; and thermal stability. A metalloenzyme character is suggested for reductase phosphatases.