Sensitivity of intestinal alkaline phosphatase to L-homoarginine and its regulation by subunit-subunit interaction

The inhibitory effect of levamisole and L-homoarginine on alkaline phosphatases (ALP, orthophosphoric monoester phosphohydrolase, EC. 3.1.3.1) in various tissues was studied to characterize differences in the mechanism of inhibition of ALP isoenzymes. The ALP activity from the placenta, kidneys, and a clonal osteogenic cell line, MC3T3-E1, was hyperbolically inhibited with a dependency on the concentration of levamisole (Ki0.5 = 10-12 microM) or L-homoarginine (Ki0.5 = 1 mM), but the activity of intestinal ALP was little inhibited by 240 microM levamisole and sigmoidally inhibited by L-homoarginine (Ki0.5 = 13 mM). Hill plot analysis of the L-homoarginine inhibition data showed that the Hill coefficient values of the placenta, kidney, and osteogenic cells were around 0.9-1.0 and that of the intestine was 1.8. The effect of sodium dodecyl sulfate (SDS), which may decrease the subunit-subunit interaction, on the L-homoarginine inhibition of the intestinal ALP was tested. The L-homoarginine concentration-dependent inhibition curve changed from sigmoidal to hyperbolic, and the Hill coefficients decreased with increasing concentrations of SDS. These results suggest that the differences in inhibition by L-homoarginine and affinity changes of intestinal ALP for L-homoarginine are caused by the subunit-subunit interaction of oligomers.