Rat liver containing radioactive native B12 was prepared by repeated injections of 57Co-OH-B12, and absorption of liver B12 was measured in patients with pernicious anemia and in subjects without stomach, using physiologic doses. It was found that absorption of liver B12 was very poor, not superior to that of free OH-B12, and coadministration of IFC markedly enhanced absorption.
In vitro digestion of rat liver with several enzymes, as determined from liberation of dialyzable radioactivity, suggested its easy digestibility. Biochemical studies of the dialyzable products of liver containing 57Co-B12 failed to demonstrate any detectable quantities of radioactivity other than free 57Co-OH-B12. A study in which cow liver powder mixed with a small quantity of 57Co-CN-B12 was fed to humans and digestion of liver was estimated from the reduction in absorption of radioactivity, indicated that most of the extractable liver B12 was liberated free in the intestine. Thus, no evidence has been obtained for the production of B12-peptide complexes from liver by digestion that require no IF for absorption.