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 (< 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