TYRPHOSTIN-23 BLOCKS PHOSPHORYLATION OF P42 BUT NOT P38 MITOGEN-ACTIVATED PROTEIN-KINASE BY ZOOXANTHELLATOXIN-A

Zooxanthellatoxin-A isolated from a symbiotic dinoflagellate, caused a ggregation in rabbit platelets that was inhibited by genistein (50 mu M) and tyrphostin 23 (500 mu M). Zooxanthellatoxin-A increased tyrosin e phosphorylation of 42-kDa proteins which were identified as p42 and p38 mitogen-activated protein kinase (MAPK) by immunoprecipitation. Ty rphostin 23 inhibited the tyrosine phosphorylation of p42 MAPK but not p38 MAPK. In contrast, genistein abolished zooxanthellatoxin-A-induce d tyrosine phosphorylation of both p42 and p38 MPSK. The results sugge st that tyrphostin 23 selectively inhibits tyrosine phosphorylation of p42 MAPK. The p38 MAPK tyrosine phosphorylation is not involved in zo oxanthellatoxin-A-induced platelet aggregation.