Influence of cobalamin deficiency compared with that of cobalamin absorption on serum holo-transcobalamin II.
BACKGROUND
Cobalamin attached to transcobalamin II (TC II), known as holo-TC II, is the active cobalamin fraction taken up by tissues. Holo-TC II is also the form in which absorbed cobalamin enters the circulation from the ileum. Therefore, holo-TC II has been proposed variously as a marker of cobalamin adequacy, cobalamin absorption, or both, including even its advocacy as a surrogate Schilling test. Such claims carry conflicting diagnostic implications because metabolic adequacy and absorption are not identical.


OBJECTIVE
The objective was to examine metabolic and absorptive influences on holo-TC II.


METHODS
Treated patients with pernicious anemia (PA), who have abnormal absorption but a normal metabolic status, were chosen as the model to differentiate between the effects of the 2 cobalamin-related characteristics. Serum holo-TC II and indexes of cobalamin metabolism in 23 treated patients were compared with those of 6 untreated PA patients (abnormal absorption and metabolic status) and 33 control subjects (normal absorption and metabolic status).


RESULTS
Holo-TC II, which correlated directly with cobalamin and inversely with homocysteine, was significantly higher in treated PA patients in metabolic remission than in untreated PA patients (74 +/- 59 compared with 9 +/- 6 pmol/L) and was significantly lower than in control subjects (105 +/- 58 pmol/L), although the latter difference was small and the values overlapped greatly.


CONCLUSIONS
Metabolic cobalamin status is a major determinant of serum holo-TC II. Absorption status may have mild influence as well, although other explanations remain possible. Serum holo-TC II cannot be used clinically to diagnose cobalamin malabsorption because of overlap with normal values. The influences on holo-TC II are complex and require careful analysis.
Chen
Xinke
X
Department of Medicine, New York Methodist Hospital, Brooklyn, NY 11215, USA.
Remacha
Angel F
AF
Sardà
M Pilar
MP
Carmel
Ralph
R
D000752:Anemia, Pernicious / Q000188:drug therapy / Q000378:metabolism*; D016022:Case-Control Studies; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D011863:Radioimmunoassay; D014155:Transcobalamins / Q000378:metabolism*; D014805:Vitamin B 12 / Q000097:blood / Q000378:metabolism* / Q000627:therapeutic use; D014806:Vitamin B 12 Deficiency / Q000097:blood / Q000175:diagnosis*
D014155:Transcobalamins; D014805:Vitamin B 12