B12 binding proteins.

A reliable sensitive method for measuring vitamin B12 in the circulation was first achieved at Hammersmith in 1950.' During the subsequent decade the vitamin B12 status was assessed of patients with a wide variety of conditions.2 This culminated in the determination of a unique function for the ileum by Booth and Mollin: that of the absorption of vitamin B123. During the course of these early studies greatly increased concentrations of vitamin B12 were found in the serum of patients with very high leucocyte counts for example, in chronic myeloid leukaemia and other myeloproliferative disorders and in occasional patients with non-leukaemic leucocytosis.4 It was shown that circulating B12 is nearly all protein bound and that raised values were associated with an increase in the serum capacity for binding the vitamin. High values for circulating vitamin B12 were also described in patients with liver cell damage including those with infective hepatitis, hepatic tumours, congestive cardiac failure and abscesses involving the liver or the subhepatic space. Booth recognised the value of these observations in assessing the patients with pyrexias of unknown cause and disturbed liver function.5 It had become clear by now that vitamin B12 required specific transport proteins for absorption from the intestine and transport in the body (Figure). The proteins fall into three groups (Tables I, II).