The binding and inhibition of trypsin by alpha-crystallin

One of the major lens-structural proteins, alpha-crystallin, is a multimeric protein containing 40 subunits of approx. 20 kDa each. There are two subunit types with distinct but similar structures. This protein was capable of inhibiting trypsin, chymotrypsin and elastase, but had no effect on thrombin or kallikrein. Complete inhibition was not observed, but rather plateau levels of inhibition were obtained in each case. Maximum inhibition was observed at a ratio of 1 mol of alpha-crystallin for every 9-10 mol of trypsin. alpha-Crystallin also inhibited the labeling of the active site of trypsin by [3H]diisopropyl fluorophosphate (DFP). Greater than 90% inhibition of DFP labeling was observed at a ratio of 1 mol of alpha-crystallin for every 7-8 mol of trypsin. Both trypsin and [3H]DFP-labeled trypsin formed a complex with alpha-crystallin, as demonstrated by gel-filtration chromatography. The active site of trypsin when bound to alpha-crystallin was still capable of reacting with p-nitrophenyl p-guanidobenzoate and soybean trypsin inhibitor, but was inaccessible to alpha 1-antitrypsin. These data suggest that alpha-crystallin acts as a multivalent modified inhibitor which is consistent with the proposed quaternary structure of alpha-crystallin.