Occurrence in Normal Human Testis of a Heat‐stable Alkaline Phosphatase Similar to a Rare Variant of Placental Isoenzyme

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Abstract
The possibility that testicular germ cells are ‘totipotential’ in expressing early developmental gene products was examined in the case of placenta‐specific heat‐stable alkaline phosphatase. In each of the 11 adult testes studied about 0.3–4.6% of the total alkaline phosphatase activity was heat‐stable and L‐phenylalanine‐sensitive but L‐homoarginine‐insensitive. However, the testicular heat‐stable enzyme was more susceptible to inhibition by L‐leucine and EDTA than the normal placental and intestinal enzymes, thus resembling the so‐called ‘D‐variant’ of placental alkaline phosphatase. By antibody‐directed enzyme inhibition tests the testicular heat‐stable enzyme crossreacted completely with normal placental enzyme but clearly distinguished itself from a heat‐stable component of normal intestinal enzyme. Thus, a minute amount of placental alkaline phosphatase D‐variant is synthesized in testis, indicating that the gene for elaborating this placental protein is probably already active in the germ cells of testis. The high incidence of this protein in cancers of testis and ovary is probably due to the increased production of this protein by active gonadal genes.