NEW KNOWLEDGE about intermediary metabolism of the imino acids in man, derived from investigations on the few patients with heritable disorders of proline and hydroxyproline metabolism, was reviewed by Efron. From an equally small number of such patients some fundamental characteristics of membrane transport of amino acids in the human kidney have also been discerned, and the majority of this discussion will be directed to this topic.
The initial identification of "iminoacidopathies" requires only simple screening of plasma or urine. The hyperprolinemias have the typical "combined" amino-aciduria1 (proline, hydroxyproline, and glycine), if the proline concentration in plasma and glumerular filtrate is sufficient (about 1μmol/ml) to saturate the "common" tubular transport system,1 and to inhibit transport of the other two substrates. Because the plasma proline level in type 1 hyperprolinemia is often less than 1μmol/ml (normal: about 0.2μmol/ml), plasma screening is theoretically more reliable than urine screening to detect