[Studies on a platelet aggregation inhibitor isolated from the human placenta].

Platelet aggregation inhibitor (PPAI) from the human placenta was partially purified and the effects of PPAI on the platelet aggregation were investigated. The results are as follows. 1) The final recovery of PPAI was 1.7% of the starting microsomal fraction. 2) The molecular weight of PPAI was estimated to be 240,000 by gel filtration on a Sephadex G-200 column. 3) PPAI was completely inactivated by heating at 70 degrees C and trypsin digestion. 4) PPAI was found to inhibit the second phase of ADP or epinephrine induced aggregation. However, it did not affect the primary phase of aggregation. The inhibitor effect was also obtained when aggregation was induced by collagen and arachidonic acid (AA). 5) Platelet secretion was also inhibited by PPAI. 6) Malondialdehyde formation was significantly reduced by PPAI. 7) PPAI inhibited the platelet cyclooxygenase (PCO) pathway: Approximately 62% inhibition of the conversion of AA into thromboxane B2 (TXB2) was observed. These results indicate that PPAI may play an important role in the platelet aggregation inhibitor, in part, by suppressing the PCO pathway.