Kinetics of monocyte 1α-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) 2D 3] levels has been attributed to impaired substrate availability to renal 1α-hydroxylase. Because serum 1,25(OH) 2D 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) 2D 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 1α-hydroxylase were 2.7- and 4-fold above normal, respectively. When serum 1,25(OH) 2D 3 of HP was corrected by intravenous 1,25(OH) 2D 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) 2D 3 levels nor on the K(m) or the V(max) of PBM 1α-hydroxylase but caused a 11-fold increase in serum 24R,25- dihydroxyvitamin D 3 [24R,25(OH) 2D 3]. In HP, 25(OH)D 3 therapy raised serum 1,25(OH) 2D 3 and reduced the K(m) and V(max) of PBM 1α-hydroxylase, which correlated negatively with serum 1,25(OH) 2D 3. However, serum 24R,25(OH) 2D 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) 2D 3, despite higher enzymatic activity; 2) 1,25(OH) 2D 3 deficiency plays a role in enhanced 1,25(OH) 2D 3 synthesis and impaired access of 25(OH)D 3 to PBM 1α-hydroxylase; and 3) abnormal 25(OH)D 3 delivery also affects 24-hydroxylation.