Familial hyperglycinuria. New defect in renal tubular transport of glycine and imino acids

Hyperglycinuria was studied in a father and his two sons. Plasma glycine concentrations were normal in all three, and although the father had a history of renal colic, his sons were healthy and asymptomatic. Urinary glycine excretion after oral or intravenous loading was increased in both sons. Intravenous proline infusion in the younger son (Case 2) showed a normal maximal transport rate (T_m) for proline, but there was marked splay in the renal tubular titration curve for proline reabsorption, consistent with a "K_m" mutation affecting proline binding. Glycine reabsorption was not further depressed by proline infusion. Intestinal glycine absorption and amino acid content in sweat were normal. The findings in this family represent a previously undescribed type of mutation affecting the renal transport system for glycine, proline and hydroxyproline. We propose that this mutation be designated, iminoglycinuria type II.