Evaluation of the contribution of intracellular protein dephosphorylation process to regulation of the contractility of mammalian smooth muscle tissues'

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In mammalian soomth muscles, micromolar concentrations of okadai acid(OA), a potent protien phosphatase ihibitor, produces an irreversible inhibitory effect on the contractility. We observed that this action of OA is accompanied by a significant decrease in myosin light-chain kinase (MLCK) activety, which is partially reversed by treatment of muscle extracts with isolated protein phosphatase 2A.OA many inhibit the activity of smooth muscle MLCK by increasing the phoisphorylation level of MLCK peortein.In human neutrophils, we examined the effect of OA on the actin assembly caused by various stimulants. OA completely abrogated action assembly induced by phorbol esters, platelet activating factor and leukotriene B4, whereas the effects of the chemotacitic peptide fMLP and opsonized zymosan were unaffected. These results suggest the existence of divergent pathways mediating the resoponse to the neutrophil stimulants.Using derivatives of OA we examined the relationship between the chemical structure of OA and its affinity to protein phosphatases. We found that modifications of some functional groups of the OA molecule resulted in marked reduction of OA to phosphatases. For example, the affinity to protein dehydrogenation of the 27-hydroxyl group of OA.The marked reduction of affinity, which is equivalent to the deference of 10-15 kJ/mol in standard free energy change, may imply that the 27-hydroxyl group serves as a binding site for the protein phosphatases.