Vitamin B 12 in the rat kidney. Evidence for an association with lysosomes

Abstract

The subcellular distribution of microbiological vitamin B 12 activity and of radioactivity was studied in the rat kidney, 2–22 days after the administration of 20 mμg 58CoB 12. The subcellular distribution of radioactivity was the same as that of the microbiological B 12 at all times. Over 50% of each was in the mitochondrial fraction at a greater concentration than in any other subcellular fraction. Investigation of the distribution of microbiological vitamin B 12 activity and of radioactivity among three mitochondrial subfractions, from 6–29 days after administration of a dose of 57Co- or 58CoB 12, showed the distributions of the two to be the same at all times. Almost 50% of the B 12 was in the lysosome-rich subfraction (M 3) at a concentration per mg protein about six times greater than in any other mitochondrial subfraction or in any of the subcellular fractions. Density-gradient centrifugation confirmed the association of B 12 with lysosomes. The M 3 subfraction was unable to take up 57CoB 12 in vitro. Using an M 3 fraction from a rat previously administered 57CoB 12, it was shown that on incubation at 37 ° radioactivity was released with similar kinetics to the release of lysosomal enzymes.