Abstract: AIM: To observe the effects of papain on monocyte-platelet aggregates (MPA) formation and monocyte activation induced by MPA, and to explore the primary mechanism.METHODS: Monocytes and platelet rich plasma(PRP) were separated from peripheral blood of volunteers. This study was assigned three groups: group 0 U/L, 20 U/L and 80 U/L of papain. Expression levels of MPA, P-selectin and CD11b after monocyte and platelet co-incubated with different concentration of papain as well as after only platelet incubated with papain, and expression levels of MPA and CD11b after only monocyte incubated with papain were measured in different groups. The inhibitory rates of MPA and CD11b expression in different concentration of papain were calculated.RESULTS: With papain concentration increasing, MPA ,P-selectin and CD11b expression levels were significantly decreased after monocyte and platelet co-incubation with papain as well as after only platelet incubation with papain(P<0.01); and MPA and CD11b expression levels were remarkably decreased, too(P<0.01). From the different treatment groups, the inhibitory rates of MPA and CD11b of group 80 U/L were notably higher than that of group 20 U/L (P<0.001). From the group of monocyte and platelet co-incubated with papain, the inhibitory rates of MPA and CD11b were markedly higher than that of other treatment groups(P<0.01).CONCLUSION: Papain is able to inhibit monocyte-platelet aggregate formation and monocyte activation induced by MPA mainly through inhibition of platelet p-selectin expression and monocyte-platelet interaction, which may be a new mechanism underlying the anti-thrombosis and anti-sclerosis effects of papain.

Key words: papain, monocyte-platelet aggregates, P-selectin, monocyte activation