Two years after they cloned the gene for a vital kidney enzyme, vitamin D-1-alpha hydroxylase, researchers at the University of California San Francisco are beginning to understand how the enzyme works. They are learning how it is regulated normally and how it acts abnormally in both a rare inherited form of rickets and in common forms of vitamin D deficiency, for example bone growth problems caused by chronic kidney failure or by aging.
Anthony Portale, M.D., professor of pediatrics and chief of pediatric nephrology at UCSF, presented these findings in an invited symposium at the Pediatric Academic Societies' annual meeting in San Francisco on Sunday, May 2. Portale and workers in the laboratory of pediatric endocrinologist Walter L. Miller, M.D., collaborated in cloning the gene for the enzyme in 1997.
The 1-alpha hydroxylase enzyme works in the kidney to perform one of the final steps in a long process that converts vitamin D -- the vitamin that comes in a morning glass of milk or is made when the skin is exposed to sunlight -- into a hormone called calcitrol. The main work for calcitrol as a hormone is to regulate the absorption of calcium from the intestine into the blood and the deposition of calcium into bone. Without it, adult bones become brittle and children's bones do not grow. The calcitrol-calcium combination also is necessary to aid nerve growth and muscle function.
"Work with the gene has provided more evidence for what we already suspected -- that production of the active form of vitamin D hormone by the kidneys is essential for normal bone health and bone growth in children," Portale said. This is a particular concern for his patients; he leads the pediatric kidney dialysis and nephrology practice for Lucile Packard Children's Hospital at UCSF. People with kidney failure do not have a genetic deficiency in vitamin D metabolism, but without normal kidney function they lose the ability to make vitamin D in its active form.