Lysine transport in human kidney

The lysine renal tubular transport was studied in a patient with an unusual trait of the membrane transport inborn error characterized by persistent hyperlysinemia with hyperlysinuria. Plasma and urine concentrations of dibasic amino acids were measured basally and at different time intervals after oral lysine loading (300 mg per kg body weight). Compared with controls, the patient's basal urinary excretions were significantly elevated, especially that for lysine. Concentrations of plasma and urine cystine in the patient were within normal ranges and were not affected with lysine loading. These data, as well as the absence of other symptoms exclude the possibility that this may be the case of cystinuria or a lysinuric protein intolerance trait. The results suggest as follows: (a) in the human kidney the tubular transport of lysine occurs via two kinetically distinct systems; (b) the high affinity lysine transport system operating at a low lysine filtered load is common to all three dibasic amino acids; (c) the low affinity lysine transport system operating at a high lysine filtered load is more specific to lysine and has a greater capacity; (d) in the patient observed the high affinity transport system is impaired for all three dibasic amino acids, especially that for lysine; the affinity for lysine is, compared to controls, about three times lower; the lysine capacity of the low affinity lysine transport system is about ten times lower than that in controls; at the same time a great amount of the patient's arginine is reabsorbed by this transport systems. The case reported indicates a clinical heterogeneity of human hereditary disorders of the membrane transport.