Differential inhibition of the products of the human alkaline phosphatase loci.

1. Inhibition studies have been carried out on a series of ALPs derived from liver, bone, kidney, placenta and intestine, using L-phenylalanine, L-homoarginine, L-leucine, L-leucyl-glycyl-glycine and L-phenylalanyl-glycyl-glycine as inhibitors. 2. No differences between liver, bone and kidney ALPs with any of the inhibitors were observed. 3. L-phenylalanine and L-homoarginine give a major degree of discrimination between liver/bone/kidney ALP on the one hand, and placental and intestinal ALPs on the other. L-leucyl-glycyl-glycine and L-phenylalanyl-glycyl-glycine give a major degree of discrimination between placental ALP on the one hand, and intestinal ALP and liver/bone/kidney ALP on the other. L-leucine discriminates between the three classes, but to a lesser degree. Minor degrees of discrimination between placental and intestinal ALPs occur with L-phenylalanine and L-homoarginine and between intestinal and liver/bone/kidney ALPs with L-leucyl-glycyl-glycine and L-phenylalanyl-glycyl-glycine. By using an appropriate combination of inhibitors the ALPs can be separated into three clearly distinct categories: placental, intestinal and liver/bone/kidney. 4. The six common placental ALP phenotypes as defined by electrophoresis show identical inhibition profiles with the series of inhibitors. The same profile was found for several rare electrophoretic variants. However, two rare electrophoretic variants (P-187 and P-92) each encountered once in a sample of 225 plancentae, showed significantly deviant inhibitions with the various inhibitors and also differed from each other. From the electrophoretic patterns, both of these rare phenotypes appear to be heterozygotes. P-187 probably corresponds to the so-called D-variant previously described. P-92 represents a new type of placental ALP variant with an aberrant inhibition profile. In both cases the particular rare allele concerned evidently alters the primary structure of the enzyme so that it has an altered electrophoretic mobility and also an altered sensitivity to inhibition for each of the different inhibitors. 5. Treatment of the various ALPs with neuraminidase to remove sialic acid residues does not affect their inhibition characteristics or activities.