Abstract

The biochemical and immunological characterization of biotinidase was performed in sera from 100 normal individuals, 68 children with profound biotinidase deficiency (less than 10% of mean normal activity) who were identified symptomatically and by newborn screening, and 63 of their parents. On isoelectric focusing, serum enzyme from normal individuals exhibits extensive microheterogeneity, consisting of at least four major and five minor isoforms at pH 4.15-4.35. Patients with profound biotinidase deficiency can be classified into at least nine distinct biochemical phenotypes, on the basis of (a) the presence or absence of cross-reacting material (CRM) to biotinidase, (b) the number of isoforms, and (c) the distribution frequency of the isoforms. None of the patients with CRM had an abnormal Km of the substrate for the enzyme. All of the parents had normal isoform patterns. The mean activities, CRM concentrations, and specific activities were not significantly different between parents of CRM-positive children and parents of CRM-negative children. There is no relationship between either the age at onset or the severity of symptoms and the isoform patterns or CRM status of the symptomatic children. The isoform patterns of children identified by newborn screening are not different from those of symptomatic children.