Lack of homology between dog and human placental alkaline phosphatases

Alkaline phosphatases [ALPases; orthophosphoric-monoester phosphohydrolase (alkaline optimum), EC 3.1.3.1] from dog and human placenta, liver, bone, kidney, and intestine were investigated by inhibition studies with L-homoarginine, L-phenylalanine, and L-phenylalanylglycyl-glycine; by thermostability studies; and by electrophoresis, both before and after treatment with neuraminidase. The inhibitions obtained for each inhibitor with dog placental ALPase closely match those obtained with dog and human liver, bone, and kidney ALPases, but are quite different from those obtained with human placental ALPase. Dog placental ALPase is thermolabile, as are dog and human liver, bone, and kidney ALPases, in marked contrast to human placental ALPase, which is very thermostable. Dog placental ALPase has the same electrophoretic mobility as dog liver, bone, and kidney ALPases after removal of sialic acid residues with neuraminidase. Desialated human placental ALPase differs electrophoretically from desialated human liver, bone, and kidney ALPases, which show the same mobilities. Dog and human intestinal ALPases are distinguished by these various criteria from the liver, bone, kidney, and placental ALPases of both species, but are similar to each other. These results suggest that the ALPase gene locus expressed in dog placenta is not homologous to that expressed in human placenta. Rather, it appears to be homologous to the ALPase locus expressed in dog and human liver and possibly also bone and kidney. Other incomplete data suggest that this may also be true for placental ALPase in other mammalian species. One possible explanation is that human placental ALPase, a relatively recent newcomer on the evolutionary scene, arose from a gene duplication that occurred subsequent to the evolutionary divergence of many other mammalian species.