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For the purpose of elucidating the physiological significance of intrinsic factor (IF) in vitamin B12 absorption, comparative studies on the adsorption of vitamin B12 to intestinal mucosa homogenate of unweaned (aged 48 hours old) and adult rats were carried out. Any material having IF activity on the intestine of the adult rat and the unweaned rat was not secreted from the stomach of the unweaned rat. This result was confirmed by the histologic findings that the chief cells of the unweaned rat, when examined on the fundus portion of the stomach, were still undeveloped. The adsorption of vitamin B12 to the intestinal mucosa homogenate of the unweaned rat seemed to be independent on the IF and the presence of Ca++ ion. The amount of vitamin B12 adsorbed to intestinal mucosa homogenate of the unweaned rat, as expressed by mg vitamin B12/kg body weight, was from 4 to 10 times larger than that of the adult rat. The high capacity of the intestinal mucosa homogenate of the unweaned rat to adsorb vitamin B12, regarded as the first step of vitamin B12 absorption, may be related to the fact that unweaned rat can absorb a large amount of vitamin B12. The small intestine of the unweaned rat appears to take indiscriminately fairly large molecules. The results of the experiments which investigated the effects of bivalent cations on the adsorption of vitamin B12 to intestinal mucosa homogenate suggested that the so-called sequential uptake might have occurred.

Comparative Studies on the Adsorption of Vitamin B<sub>12</sub> to Intestinal Mucosa Homogenate of Adult and Unweaned Rats