Calcium-independent phospholipases from guinea pig digestive tract as probes to study the mechanism of lipocortin.

Two calcium-independent phospholipases isolated from guinea pig pancreas (lipase Ia, 37 kDa) and from guinea pig intestine (phospholipase B, 97 kDa) have been used to probe the mechanism of phospholipase inhibition by lipocortin. In the presence of calcium, both enzymes were inhibited by lipocortin I in a manner very similar to the inhibition of pig pancreas phospholipase A2. By using phospholipases that lack a requirement for calcium, we have for the first time been able to dissociate enzymatic activity from the role of calcium in the inhibitory process. It was found that lipocortin was without effect against phospholipase A1 and phospholipase B in the absence of calcium, under which conditions the inhibitory protein is unable to interact with anionic phospholipid surfaces. The same behavior toward phospholipase A1 was observed with two other related proteins, endonexin II or lipocortin V, and p68/67-kDa calelectrin or lipocortin VI. Together with the observation that lipocortins are active only in the presence of limited amounts of substrate, these data give further support to the "surface depletion model" of lipocortin inhibition, rather than to a mechanism involving a direct interaction between enzyme and inhibitor.