In inborn errors where the defective enzyme had a cofactor requirement it was found that the administration of large amounts of the vitamin resulted in a clinical and biochemical improvement. A mutation that imposes an exaggerated requirement for a vitamin affects the apoenzyme directly (pyruvate dehydrogenase deficiency, maple syrup urine disease, homocystinuria, etc.), the conver sion of a vitamin into coenzyme (methylmalonic acidemia, vitamin D-dependent rickets type 1, etc.) , or the attachment of vitamin to the apoenzyme (holocarboxylase synthetase (HCS)deficiency). Direct proof has so far been provided that molecular defects in the said gene underlie both the vitamin-responsive and vitaminmonresponsive forms. HCS deficiency is an exception, all HCS-deficient patients being responsive to biotin administration. The mutations identified in the HCS deficient patients were really responsible for their reduced HCS activity. We have proposed the concept that not only the Km of HCS for biotin, but also the V_<max> are important factors in determining the responsiveness to biotin therapy.
