Boronic acid inhibitors of porcine pancreatic lipase.

Porcine pancreatic lipase was inhibited by alkane and arene boronic acids. The inhibition by octadecane boronic acid was competitive when measured against the hydrolysis of dissolved tripropionin in the presence of siliconized glass beads. The value of Ki in this system was 1.34 x 10(3) molecules micron-2. The ratio of substrate to inhibitor concentrations giving 50% inhibition was in the range of 700 to 2200, indicating that lipase has a greater affinity for boronic acids than for tripropionin. Boronic acids did not interfere with the interaction of lipase with the siliconized glass/water interface, demonstrating that the binding of lipase to substrate interfaces, the first step in lipase action, was not the step at which inhibition occurred. The boronic acid binding site on lipase is at or near the active center serine since modification of this residue by diethyl p-nitrophenyl phosphate was prevented by boronic acids. Modification of the active center serine residue by diethyl p-nitrophenyl phosphate also prevented boronic acid binding. Binding of a chromophoric boronic acid, 7-nitrobenzo-2-oxa-1,3-diazolyl m-aminobenzene boronic acid, to lipase was demonstrated by equilibrium gel filtration on polyacrylamide beads (Bio-Gel P-60) in the presence of 4 mM sodium taurodeoxycholate. The complex contained 1 molecule of boronic acid per molecule of lipase and had a dissociation constant Kd of 5 x 10(-6) M. The boronic acid was not bound in the absence of taurodeoxycholate. Boronic acids are believed to be analogs of the tetrahedral intermediate in the action of lipase.