Biological variation of holo-transcobalamin in elderly individuals.

Abstract

Vitamin B12 is a water-soluble molecule essential for mammalian intracellular metabolism. Its two metabolically active forms, methyl-cobalamin and 5-deoxyadenosylcobalamin, are coenzymes in the reactions catalyzed, respectively, by methionine synthase and methylmalonyl-CoA mutase. There are two vitamin B12 carrier proteins in serum, haptocorrin and transcobalamin (TC). Haptocorrin binds the majority of serum B12 but, unlike TC, does not deliver the vitamin to metabolically active cells. Only 5–20% of serum B12 is bound to TC as “holo-TC”. Current laboratory assays determine total serum B12 concentrations and are relatively poor indicators of the ability of serum to deliver the vitamin to tissues. Methods are now available to measure holo-TC in clinical samples (1)(2)(3). Although information exists for “between-person” variations in holo-TC concentrations, (2)(4), very few data exist regarding its “within-person” variability (5). Such knowledge will be essential for studies of diseases potentially associated with low concentrations of holo-TC, such as Alzheimer disease (6). We therefore examined the between- and within-person variability and within-assay variability of holo-TC concentrations in healthy elderly volunteers in the fasting and nonfasting states. The study received local research ethics committee approval and followed an established protocol aimed at minimizing various preanalytical factors that can influence the results of clinical laboratory tests (7). Because valid estimates of the components of variation can be obtained from a relatively small number of participants (7), six males and six females age ≥65 years were recruited. Their mean age was 82.5 years (range, 65–99 years). Ages were not significantly different between males and females (Student t- test, t 10 = 1.4; P = 0.2). The participants were all maintaining their usual lifestyles and not taking any medication. Ten samples of venous blood were collected at 14-day intervals from each participant over a 5-month …