Abstract

The protein kinase C (PKC) inhibitor sphingosine induces phosphorylation of an 18-kDa protein in Jurkat T cells and of a 19-kDa protein in bovine mammary gland, and suppresses phosphorylation of substrate proteins for PKC. Melittin, a toxic amphiphilic peptide from bee venom known to inhibit PKC activity, was examined to determine whether it, like sphingosine, induced phosphorylation of the 19-kDa protein. Melittin inhibited PKC activity in both cytosolic and total particulate fractions of bovine mammary gland, with IC 50 values (concentrations causing 50% inhibition) of 5–7 μM. Melittin suppressed phosphorylation of such PKC substrate proteins as a 91-kDa protein in cytosol and a 36-kDa protein in the particulate fraction. Besides the suppression, melittin induced phosphorylation of cytosolic 105-kDa, 94-kDa, 27-kDa, 24-kDa and 19-kDa and particulate 110-kDa, 53-kDa and 43-kDa proteins, which were not clearly observed in the absence of melittin. The induction could be detected at a 10 μM concentration. Phoshorylation of these proteins was reversed by excess addition of the PKC cofactor phosphatidylserine, but not by other cofactors such as oleoyl-2-acetyl- sn-glycerol or Ca 2+. PKC inhibitors other than melittin [1-(5-isoquinolinylsulfonyl)-2-methyl-piperazine, gossypol, palmitoylcarnitine and adriamycin] had little effect on the induction, except at a high concentration (500 μM) of adriamycin. These results suggest that melittin, like sphingosine, has both suppressive and stimulatory effects on protein phosphorylation in bovine mammary gland.

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