A new case of deficiency of the R binder for cobalamin, with observations on minor cobalamin-binding proteins in serum and saliva

A patient presented at the age of 77 yr with a low serum cobalamin level. Subsequent study showed that he had persistently very low R binder (TC I) cobalamin-binding capacity in serum (less than 5 ng/liter versus 213 +/- 171 ng/liter in normal controls), and that almost all of his endogenous serum cobalamin was carried by TC II instead of TC I. His saliva also demonstrated virtually undetectable R binder (binding capacity of 31–38 ng/liter versus 41,690 +/- 23,820 ng/liter for control subjects). Unlike previous cases of R binder deficiency, he seemed to maintain normal serum cobalamin levels while receiving monthly cyanocobalamin injections. This and his normal serum unsaturated binding capacity were due to elevated TC II levels. TC II carried 72%-98% of his endogenous cobalamin, the rest being attached to minor binders. As incidental findings, the patient had a serum component of molecular weight of approximately 70,000 that carried 7%- 8% of his endogenous cobalamin and also had small quantities of TC II demonstrable in his saliva. Both these heretofore unappreciated minor peaks were identifiable because of the lack of R binder. The patient's clinical presentation supports the conclusion that R binder deficiency is a benign disorder. Whether his mild hypersegmentation of neutrophils and neuropathy were related to the R binder deficiency or, more likely, arose from coexisting folate deficiency and alcohol abuse, the overall picture contrasts dramatically with the severe clinical sequelae of TC II deficiency.