Presence of the rare D-variant heat-stable, placental-type alkaline phosphatase in normal human testis.

In 11 adult testes studied, about 0.3 to 4.6% of the total alkaline phosphatase activity was heat stable and L-phenylalanine sensitive but L-homoarginine insensitive. The testicular heat-stable enzyme was more susceptible to inhibition by L-leucine and ethylenediaminetetraacetate than were the normal placental and intestinal enzymes. By antibody-directed enzyme inhibition test, the testicular heat-stable enzyme cross-reacted completely with normal placental enzyme but clearly distinguished itself from a heat-stable component of normal intestinal enzyme. Thus, placental alkaline phosphatase D-variant is synthesized in testis, indicating that the gene for elaborating this placental protein is probably already active in the testicular cells. The high incidence of this protein in cancers of testis and ovary is probably due to its increased production by gonadal genes present in the genome of these particular tumors.