Calcium-dependent cyclic nucleotide phosphodiesterase inhibition of basal activity by heat-stable factors from rat cerebrum

Abstract

The boiled supernatant fraction from rat cerebrum contained factors which inhibited the basal activity of a Ca 2+-dependent phosphodiesterase from rat cerebrum. Two inhibitory fractions were isolated by DEAE-cellulose or Sephadex chromatography and were deemed proteins, based on their sensitivity to trypsin digestion. The inhibitory fractions eluted from DEAE-cellulose columns prior to the Ca 2+-dependent activator protein. The inhibitory factors, unlike the activator protein, were stable to heat treatment under alkaline conditions. The inhibitory factors caused both an increase in K m for cyclic GMP and a decrease in V . In the presence of calcium ions and purified activator protein, the Ca 2+-dependent phosphodiesterase was not inhibited by the factors, but instead was slightly stimulated. The inhibitory factors caused a slight apparent stimulation of a Ca 2+-independent phosphodiesterase from rat cerebrum but this proved instead to be a nonspecific stabilizing effect which was mimicked by bovine serum albumin. After prolonged alkaline treatment, the purified activator protein caused a modest Ca 2+-independent activation of Ca 2+-dependent phosphodiesterase. The inhibitory factors antagonized the activation of Ca 2+-dependent phosphodiesterase by alkaline treated activator protein or by lysophosphatidylcholine. The inhibitory factors had no effect on activity of trypsinized Ca 2+-dependent phosphodiesterase. Of various other proteins, only casein mimicked the effects of the inhibitory factors on phosphodiesterase activity.