KINETICS OF MONOCYTE 1-ALPHA-HYDROXYLASE IN RENAL-FAILURE

In chronic uremia, the requirement of supraphysiological doses of serum 25-hydroxyvitamin D-3 [25(OH)D-3] for the normalization of 1,25-dihydroxyvitamin D-3 [1,25(OH)(2)D-3] levels has been attributed to impaired substrate availability to renal 1 alpha-hydroxylase. Because serum 1,25(OH)(2)D-3 can also be corrected by 25(OH)D-3 supplementation in bilaterally nephrectomized patients, we examined the role of substrate availability on 1,25(OH)(2)D-3 production by peripheral blood monocytes (PBM). In hemodialysis patients (HP), 25(OH)D-3 uptake was 50% lower than normal, and the maximal velocity (V-max) and apparent Michaelis constant (K-m) for 25(OH)D-3 of 1 alpha-hydroxylase were 2.7- and 4-fold above normal, respectively. When serum 1,25(OH)(2)D-3 of HP was corrected by intravenous 1,25(OH)(2)D-3, 25(OH)D-3 uptake, K-m, and V-max returned to normal values. The effect of 25(OH)D-3 supplementation was also examined. In normal adults, 25(OH)D-3 administration had no effect on serum 1,25(OH)(2)D-3 levels nor on the K-m or the V-max of PBM 1 alpha-hydroxylase but caused a Ii-fold increase in serum 24R,25-dihydroxyvitamin D-3 [24R,25(OH)(2)D-3]. In HP, 25(OH)D-3 therapy raised serum 1,25(OH)(2)D-3 and reduced the K-m and V-max of PBM 1 alpha-hydroxylase, which correlated negatively with serum 1,25(OH)(2)D-3. However, serum 24R,25(OH)(2)D-3 only increased slightly above basal. These results demonstrate that, in HP, 1) impaired uptake of 25(OH)D-3 and low affinity for substrate determine the need for high 25(OH)D-3 levels to normalize serum 1,25(OH)(2)D-3, despite higher enzymatic activity; 2) 1,25(OH)(2)D-3 deficiency plays a role in enhanced 1,25(OH)(2)D-3 synthesis and impaired access of 25(OH)D-3 to PBM 1 alpha-hydroxylase; and 3) abnormal 25(OH)D-3 delivery also affects 24-hydroxylation.