Effects of polyamines on proline endopeptidase activity in rat brain

The in vitro effects of polyamines on the activity of proline endopeptidase (PEPase) in rat brain cytosol, which contains an endogenous PEPase inhibitor, have been studied. Of the three amines tested (spermine, spermidine, and putrescine), spermine and spermidine markedly enhanced the enzyme activity in brain cytosol. At 6.25 mM spermine or 25 mM spermidine, a 13- or 14-fold enhancement of the enzyme activity was observed. When Mg2+ was used, an approximately fourfold enhancement of the enzyme activity was observed at 50 mM. The enhancement produced by spermine or spermidine was unaffected by Mg2+ up to 50 mM. The activity of purified PEPase was only slightly affected by each polyamine, but it was inhibited 50% by 50 mM Mg2+. On the other hand, 50% inhibition of the enzyme produced by the purified PEPase inhibitor (Mr 7,000: Ki 0.67 mM) was completely restored by addition of 0.7 mM spermine, 3.5 mM spermidine, or 28 mM putrescine. This restoration of inhibition by polyamines was reversed by increasing the inhibitor concentration. These data suggest that polyamines effectively reverse the inhibition of PEPase by its endogenous inhibitor by the reversible formation of a kinetically significant complex. The possible functions of polyamines in the regulation of PEPase in vivo are discussed.