However, 1 h after ingestion of 5 J. lg/kg of biotin, her plasma biotin concentration rose to 5650 pg/ml, as in similarly treated biotin-replete controls. Biotin clearance was also normal (34-4 4 ml/min/1’73 m2). The fraction of biotin excreted in her urine in the 24 h after the biotin dose was 50’8%, twice as high as the amount excreted by the controls and more than three times her previous maximum excretion (15%). Biotinidase activity in the patient’s serum was undetectable (table). The mother, who does not have clinical biotin deficiency, had 46% of mean normal biotinidase activity and a plasma biotin of 248 pg/ml, which is below normal. Dietary biotin exists primarily as biotinyl groups linked to the E-amino groups of lysine residues of proteins. 9 Biotinidase hydrolyses this bond, releasing free biotin. Biotinidase activity is widely distributed, particularly in liver, kidney, serum, and gut mucosa. 9 Although this enzyme’s primary site of action on dietary protein is not known, deficiency could impair ability to maintain normal body stores of free biotin, leading to a hypobiotinaemic state. Whether biotinidase is also involved in the absorption of free biotin is not known.

The patient’s normal plasma biotin response to the ingestion of 5pg/kg of biotin when her tissues were replete with biotin, but her failure to respond when she was biotin deficient is probably caused by rapid entry of biotin into the depleted tissues. Similar results