Modulation of phospholipase A2 activity in extracts of lesion-free psoriatic epidermis by alkaline phosphatase and a protein phosphatase inhibitor.

Phospholipase A2 activity is raised in non-lesional psoriatic epidermis compared with normal epidermis. It has been shown that the activity of this enzyme is controlled by an inhibitory protein the inhibitory effect of which is increased by dephosphorylation. Treatment of epidermal extracts with alkaline phosphatase reduced the phospholipase A2 activity, both in normal and in lesion-free psoriatic epidermis. Inclusion of pyrophosphate, a protein phosphatase inhibitor, in the homogenizing medium caused the activity of phospholipase A2 in epidermal extracts from normal and lesion-free epidermis to be raised to the same high level. These results are consistent with the hypothesis that the raised phospholipase A2 activity in psoriatic epidermis is due to hyperphosphorylation of an endogenous inhibitor as a result of defective control of a phosphorylation/dephosphorylation mechanism. The relevance of these findings to other work is discussed.