Background: Evolutionary haptocorrin is the youngest of the cobalamin-binding proteins. It evolved by duplication of the intrinsic factor gene and has been identified in most mammals examined. Its ability to bind both cobalamin and analogues is well established, but apart from that, our knowledge concerning its function and its distribution in adult and foetal life is limited. In this study, we present data on the tissue expression of haptocorrin and on the relation between analogues on haptocorrin and vitamin B12 status in humans.
Methods: Polyclonal antibodies towards haptocorrin were used to study the localisation in foetal and adult tissues by immunohistochemistry. Positive immunoreactions were primarily observed in exocrine glands, the gastrointestinal tract and the respiratory system. ELISA was used for measurement of holo- and total haptocorrin in blood samples from individuals diagnosed with vitamin B12 deficiency, based on measurement of methylmalonic acid (μmol/L) as evident (>0.75, n=61), suspected (0.29–0.75, n=155) or not present (<0.29, n=170). Cobalamins and holotranscobalamin were measured in the same individuals.
Results: Holohaptocorrin was considerably higher than holohaptocorrin-cobalamins (cobalamins minus holotranscobalamin). The median (25th–75th percentile, pmol/L) for holohaptocorrin analogues (holohaptocorrin minus holohaptocorrin-cobalamins) was higher in deficient [200 (130–240)] compared to the non-deficient [140 (80–200)] individuals (analysis of variance and Tukey's multiple comparison test, p<0.01).
Conclusions: Our results indicate that haptocorrin is widely distributed also in foetal tissues and suggest analogues to accumulate on haptocorrin in vitamin B12-deficient individuals, a result that warrants further studies employing methods directly measuring cobalamins and analogues attached to haptocorrin.
Clin Chem Lab Med 2007;45:1751–9.