Determinants of variation in plasma alkaline phosphatase activity: a twin study.

Plasma alkaline phosphatase activity has been measured in 204 pairs of twins aged 18-34. Estimated genetic variance was the same in men and women, after exclusion of the youngest men, but environmental variance was greater in men so that the heritability was .90 +/- .02 in the women and .71 +/- .05 in the 19-34-year-old men. About 15% of the genetic variance was associated with the ABO blood group polymorphism, which is known to affect intestinal alkaline phosphatase. Differences in plasma alkaline phosphatase activity between normal people may be mainly due to genetically determined proportions of isoenzymes of differing stability.