Interaction of concanavalin A with membrane-bound and solubilized lipoprotein lipase of rat heart.

Concanavalin A was used to study the configuration of lipoprotein lipase at the surface of capillary endothelium. Incubation of heart homogenates with increasing concentrations of concanavalin A for 5-60 min resulted in inhibition of up to 50% of enzyme activity. The inhibition was related to the concentration of lectin and the time of incubation and was fully reversible by postincubation with alpha-methyl-D-mannoside. Rat hearts were perfused for 5-60 min and lipoprotein lipase activity determined in postheparin perfusates and in the perfused heart. When the lectin was introduced into the perfusate a significant reduction of heparin-releasable enzyme was found after 30 min of perfusion. The missing enzyme could be recovered by postperfusion with alpha-methyl-D-mannoside, but not by addition of the sugar to the perfusate withdrawn from the apparatus. These results suggested binding of lectin to the surface-located enzyme and support for such a binding was obtained by the finding of release of labeled lectin into the perfusate by heparin. Perfusion of hearts with concanavalin A for 60 min resulted also in a fall in nonreleasable lipoprotein lipase. The mechanism of this fall is not due to impairment of enzyme synthesis, as leucine incorporation into protein was not reduced. Since neither perfusion nor postincubation with alpha-methyl-D-mannoside restored enzyme activity, the fall was most probably due to irreversible inhibition. It is concluded that mannose residues of lipoprotein lipase in heart homogenates and at the endothelial surface of heart capillaries are available to interact with a specific lectin. Such an interaction renders the enzyme less releasable by heparin during perfusion and causes a significant inhibition of enzyme activity in homogenates.