Absorption, Plasma Transport, and Cellular Retention of Cobalamin Analogues in the Rabbit: EVIDENCE FOR THE EXISTENCE OF MULTIPLE MECHANISMS THAT PREVENT THE ABSORPTION AND TISSUE DISSEMINATION OF NATURALLY OCCURRING COBALAMIN ANALOGUES

Abstract Analogues of cobalamin (Cbl; vitamin B12) are prevalent in nature as a result of bacterial synthesis, and are of additional interest because of their potential use as antimetabolites and chemotherapeutic agents. We have synthesized 14 Cbl analogues containing 57Co and have compared their gastrointestinal absorption, plasma transport, and cellular retention to that of [58Co]Cbl in rabbits. Many of the Cbl analogues were bound with low affinity by intrinsic factor, and none of these [57Co]Cbl analogues were taken up by the ileum or absorbed into the body in amounts comparable to that of [58Co]Cbl. The Cbl analogues that were bound by intrinsic factor with high affinity were taken up by the ileum but, in many cases, they were retained there in significant amounts. Most of the Cbl analogues were bound by plasma transcobalamin II with high affinity and all of these transcobalamin II-[57Co]Cbl analogue complexes were taken up by a variety of tissues in a manner that was indistinguishable from that of transcobalamin II-[58Co]Cbl. The few analogues that were bound by transcobalamin II with low affinity were taken up by tissues in lesser amounts, and 20-70% of these analogues was rapidly excreted in the urine as occurs with native Cbl when it is present in plasma in unbound form. All of the Cbl analogues were bound by the granulocyte R-type Cbl-binding protein with high affinity and all of the R-type protein-[57Co]Cbl analogue complexes were cleared rapidly from plasma exclusively by hepatocytes as occurs with R-type protein-[58Co]Cbl. Some Cbl analogues were released back into the plasma and were disseminated among a variety of tissues via transcobalamin II as occurs with native Cbl. Other Cbl analogues were retained in the liver and eventually excreted in the feces and urine without accumulating in other tissues. These studies indicate that intrinsic factor and the ileum prevent certain Cbl analogues from entering the body and that the granulocyte R-type protein and hepatocytes prevent the dissemination of certain Cbl analogues that may gain entry such as during infections with Cbl analogue-producing bacteria. The fact that transcobalamin II binds and transports a large number of Cbl analogues indicates that these protective mechanisms can be circumvented and supports the feasibility of using Cbl analogues as antimetabolites in vivo.