Metabolism and uptake of l-proline by human kidney cortex.

Abstract

Extract: L-Proline transport and metabolism have been investigated in normal adult human kidney cortical slices. The uptake of proline from buffers containing both high (10 mM) and low (0.23 mM) 14C-proline concentrations is intimately related to the extent of intracellular proline metabolism. The accumulation of radioactivity from both the 10 and 0.23 mM proline media was against high radioactivity gradients, with distribution ratios of 18 and 3, respectively. Recovered proline from the intracellular fluid (IGF) accounted for less than 12% of the intracellular radioactivity in the tissue extracts. Glutamic acid accounted for greater than 75% of the intracellular form of the soluble radioactivity in tissue extracts. Total oxidation to 14CO2 from both proline concentrations accounted for more than 50% of the total proline taken up by the slices.Kinetic analysis of the entry process suggested the existence of two saturable systems: one operative at low or physiologic proline concentrations (apparent Km 2 mM) with a low capacity (Vmax 40 μM-ml ICF-1-30·min-1) and shared with neutral amino acids, and a second with an affinity to 10-fold less (apparent Km 17 mM) with a high capacity (Vmax 160 μM·ml ICF-1·30 min-1) and unshared by the neutral amino acids. Separation of the influence of metabolism from the analysis of the transport systems by the use of proline analogues showed that thioproline inhibited proline uptake by 80%, primarily by reducing the conversion of proline to glutamic acid, whereas 3,4-dehydroproline did not affect the conversion of proline to glutamic acid but principally appeared to alter the affinity of the transport system for proline.Speculation: These studies suggest that changes in the rate of proline metabolism in the human kidney may alter the rate of tubular reabsorption, thus controlling renal proline excretion. It is possible that renal iminoglycinuric syndromes may not only be secondary to membrane transport disorders but may result from localized metabolic defects in the tubule cells.