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Skin histidase activities and urine formiminoglutamic acid (FIGLU) levels were measured in 20 patients with histidinemia, identified by Guthrie's screening method, and their family members as well as control subjects. There was a significant positive correlation between skin histidase activities and the amounts of urine FIGLU. Although the difference of skin histidase activity and the amount of urinary FIGLU was significant between any two of the three groups (i.e. controls, parents and patients; p less than 0.005), these levels ranged widely and a considerable number of the cases overlapped among groups. When a discriminant function was computed to obtain the minimum probability of misclassification between the groups using the above two parameters, a better segregation was observed. However, even though the number of misclassifications decreased, the overlapping cases were still present, especially between the parent and patient groups. It is concluded that either skin histidase activity, urine FIGLU, or both, can be used as genetic markers of the disease to a large but still limited extent.

A new sensitive method was developed for assay of histidase in the stratum corneum of human skin with [ 14 C]histidine as substrate. With this method histidase could be detected using less than 1 mg of stratum corneum. Skin histidase was markedly reduced in seven infants with histidinemia. The enzyme activities in thirteen parents of the patients were about 50% of the normal values, and even the highest values in the parents did not overlap the lowest normal values. Thus heterozygotes in the autosomal recessive form of histidinemia can be distinguished from normal persons by their skin histidase activity measured by this sensitive assay method.