Holocarboxylase synthetase (HCS) deficiency is a metabolic disorder that causes a biotin-responsive multiple carboxylase deficiency. We analyzed the kinetic properties of seven mutant HCS proteins. Two of these enzymes harbored mutations within the putative biotin-binding region of HCS and showed elevated K(m) values for biotin compared with that of the wild-type form (K(m) mutant; Gly581Ser: 45 times, delThr610: 3 times). The remaining five mutations (Arg183Pro, Leu216Arg, Leu237Pro, Va1333Glu, and Va1363Asp) were located outside the biotin-binding region. The enzymes containing these mutations showed normal or low K(m) values for biotin (non-K(m) mutant). Symptoms of patients who have the non-K(m) mutants, as well as those of patients who have the K(m) mutants, responded to biotin therapy. This is probably because the K(m) value for biotin of normal HCS is higher than the physiologic concentration of biotin in human cells. The V(max) values of all mutant HCS proteins were considerably decreased, but to a variable degree. The responsiveness to biotin supplementation of propionyl-CoA carboxylase activity in cultured cells beating the mutations correlated well with the degree of reduction in the V(max) of HCS. Patients who have mutant HCS proteins with lower V(max) showed poorer clinical and biochemical responses to biotin therapy. These observations suggest that the reduction of V(max) is an essential factor for pathophysiology and prognosis of HCS deficiency under treatment with large amounts of biotin. The determination of HCS genotype can be valuable for characterizing the clinical phenotype in HCS deficient patients.
ASJC Scopus subject areas
- Pediatrics, Perinatology, and Child Health