Oligosaccharides of human milk. Isolation and characterization of three new disialyfucosyl hexasaccharides.

Renal mitochondrial 25-hydroxyvitamin D₃-1-hydroxylase (1-hydroxylase) is sensitive to inhibition by 2 × 10^?5m calcium and 5 × 10^?3m phosphate when hydroxylation is supported by either malate or NADPH. This sensitivity to ion inhibition is observed in mitochondria from both vitamin D-deficient and repleted chicks and remains when mitochondria are frozen and thawed or are incubated in a hypotonic medium. The ionophore A23187 inhibits the 1-hydroxylase but partially reverses the inhibition exerted by 2, 5, or 7.5 × 10^?5m calcium. Addition of a kidney soluble cell fraction (37,000g supernatant) to isolated mitochondria did not enhance the 1-hydroxylase activity under conditions of varied substrate concentration, osmolarity of the incubation medium, or mitochondrial washes. It is concluded that a soluble cellular component is not involved in the regulation of the 1-hydroxylase but that intramitochondrial calcium and phosphate may well play a role in its regulation.