Mixed-function oxidases of 25-hydroxycholecalciferol in isolated chick kidney glomeruli: evidence for nuclear localization.

Pure chick kidney glomeruli and proximal tubular fragments have been isolated by graded sieving through nylon screens. Electron micrographs revealed that, in distinct contrast to proximal epithelial cells, the glomerular epithelial and endothelial cells are essentially devoid of mitochondria. Glomeruli as well as proximal tubular fragments contain the 1 alpha- and 24R-hydroxylases of 25-hydroxycholecalciferol. The level of 1 alpha-hydroxylase activity was the same in both segments of the nephron. However, the tubular fragments contained twice the 24R-hydroxylase activity found in glomeruli. Glomerular nuclei were purified by sucrose gradient sedimentation and used to confirm the association of the 1 alpha-hydroxylase with this kidney organelle. Almost all of the glomerular 1 alpha-hydroxylase activity was found in the nuclear fraction. Two metabolites, which are produced predominantly by the nuclei, are designated N-1 and N-2. Their structural identity remains unknown. The novel presence of the 1 alpha-hydroxylase in the glomerulus may be important in defining the etiology of bone diseases in patients with glomerulonephritis and similar disorders.