The action of a new anticancer agent, the semisynthetic alkyl-phospholipid plasmanyl-(N-acyl)-ethanolamine (sPNAE), namely 1-O-octadecyl-2-oleoyl-sn-glycero-3-phospho-(N-palmitoyl)-ethanolamine, on protein kinase C (PKC) was investigated, and it was found to inhibit in a dose-dependent manner PKC isolated from mouse brain. The inhibition was competitive with respect to phosphatidylserine (K(i) = 20 microM). Lyso-PNAE, a possible cell metabolite of sPNAE, also inhibited PKC. A two-site model was used to calculate the binding affinity and the number of binding sites for phorbol ester in a culture of human melanoma BRO cells. The values of Kd, the dissociation constant, were K'd = 0.5 nM and K"d = 72 nM, whereas the values of Bmax, the number of binding sites, were B'max = 4.6 x 10(4) sites cell-1, and B"max = 2.9 x 10(5) sites cell-1. sPNAE was able to reduce the affinity of BRO cells for phorbol ester with almost no changes in the number of binding sites: K'd = 1.6 nM, K"d = 557 nM, and B'max = 4 x 10(4), B"max = 1.9 x 10(5). These data suggest that sPNAE may inhibit PKC in intact cells. Since various inhibitors of PKC may enhance the antiproliferative activity of cis-diamminedichloroplatinum(II) (cis-DDP), we investigated the effect of the combination of sPNAE and cis-DDP on the proliferation of BRO cells. sPNAE synergistically enhanced the antiproliferative activity of cis-DDP.