Intestinal phosphate transport in familial hypophosphatemic rickets

The present report outlines an attempt to characterize inorganic phosphate uptake by human jejunal mucosa using biopsy material obtained from six patients affected by the X-linked form of vitamin D-resistant rickets and six control subjects. The tissue from control subjects accumulated 32P actively in a linear fashion against time. The incorporation of inorganic phosphate into organophosphate derivatives is rapid and equilibrates after 10 min at an inorganic over total phosphate ratio of 0.45. Concentrative uptake and incorporation were both suppressed by anaerobiosis or cyanide. Rates of phosphate uptake and incorporation into the organic derivatives by the tissue of hypophosphatemic patients are comparable with normal values. Saturation kinetics observed over a wide range of substrate concentrations (0.003 to 3 mM) elicits only one transport system with an apparent Michaelis constant of 0.2 mM and a maximum velocity of 0.7 mmol/liter/40 min. The kinetic data obtained from the patients do not strikingly differ from the control values. The chemical analysis of the phosphate content of intestinal mucosa from two patients and two control subjects indicates that the tissue is not specifically phosphate-depleted in the mutant individuals. These results are in accordance with the positive results obtained with the oral replacement therapy in familial hypophosphatemia.