Diaphorase Activity and Variants in Normal Adults and Newborns

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Abstract
Summary. Red cell NADH‐dependent methaemoglobin reductase (diaphorase) activity was estimated in 126 normal healthy adults and 556 newborns of the three major racial groups in Malaysia. The mean enzyme activity in adults was 3.34 units/mg Hb (SD ± 0.63) and in newborns 1.75 u/mg Hb (SD ± 0.48). The difference between the two means is statistically highly significant. Indians, both adults and infants had the lowest mean enzyme activity. Mean activity in a group of newborns whose birth weights were 2.7 kg or more was significantly higher than in a group with lower birth weights. Sixteen newborns had enzyme activity of less than 1 u/mg Hb. Two of them each had one parent whose enzyme level was also very low. Both parents of three others had normal enzyme activity. The lower limit of normal in newborns seemed to overlap with that found in subjects with hereditary methaemoglobin reductase deficiency. One newborn with an extremely low enzyme level had methaemoglobinaemia, but its genetic nature could not be proved because the parents were not available for study. Examination of 108 adults and 527 newborns for diaphorase electrophoretic variants revealed two different and unusual phenotypes in a few Chinese and Indians. Family studies showed that they are genetically determined.