section 38.2
Water-Soluble Vitamins
921
appears to be released from leukocytes during sample
collection.
Normal plasma concentration of cobalamins is
200-1000 pg/mL, of which more than 90% is bound
to TCI, mostly as methylcobalamin. A serum
concentration of less than 100 pg/mL is usually
diagnostic for vitamin B
12
deficiency, but pronounced
changes in concentrations of circulating cobalophilins
could cause misleading changes in serum B
12
concentration, which is usually measured by a
competitive protein binding assay. If cobalophilins are
used as the binding protein, falsely high levels of
“vitamin B
12
activity” may be obtained because of the
presence of inactive cobalamins in the sera of some
patients and the relative nonspecificity of
cobalophilins toward corrinoids. This result is avoided
by using microbiological assays or by, using gastric
IF as the binding protein because it is more specific.
TCI has a high affinity for and is about 90%
saturated with cobalamin, while TCIII binds less
strongly and is largely unsaturated. However, TCII
binds newly absorbed cobalamins, and the
TCII-cobalamin complex appears to deliver
cobalamin to peripheral tissues (Figure 38-18). TCII
accounts for most of the unsaturated vitamin Bj
2
binding capacity of serum. Therefore, cobalophilins
(except TCII; see below) probably have a minimal
role in cobalamin absorption, transport, and
metabolism. Thus, members of the family with
congenital cobalophilin deficiency showed no
symptoms of vitamin B
12
deficiency despite a very
low serum concentration of the vitamin secondary to
the absence of TCI.
2.
Intrinsic Factor (IF).
IF is a glycoprotein (M.W.
~45,000), approximately 15% of which is
carbohydrate, including sialic acid. It binds one
cobalamin per IF molecule. The B-corrinoid ring and
the benzimidazole ring (Figure 38-15) are needed for
optimal binding. It is synthesized in the gastric
parietal cells, which also secrete HC1. The amount of
IF normally secreted each day is sufficient to bind
40-80 /xg of vitamin B
12
. Secretion, and perhaps
synthesis of IF is increased by vagal stimulation or by
histamine, gastrin, or gastrin pentapeptide. In the
small intestine, free IF is partially degraded by pepsin
and chymotrypsin but is resistant to trypsin. The
IF-B
12
complex is resistant to all of these proteases.
Pernicious anemia is a megaloblastic anemia
caused by malabsorption of vitamin B
12
secondary to
inadequate secretion of normal IF. The word
“pernicious” reflects the unremitting and usually fatal
course of the disease prior to the discovery that it
could be corrected by administration of vitamin B
12
.
Virtually all cases result from atrophic gastritis,which
in turn may result from autoimmune attack of gastric
parietal cells, leading to absence of IF and
achlorhydria.Pernicious anemia may rarely be due to
an inherited defect in structure or secretion of IF.
Several cases of congenital absence of IF have been
described. Anemia due to inborn errors usually
manifests itself early in life, while the average age
for appearance of atrophic gastritis is 60 years; the
latter condition is rare, but not unknown, before the
age of 30.
The Schilling test differentiates pernicious anemia
from other causes of megaloblastic anemia. Vitamin
B
12
containing radioactive cobalt is administered
orally, and a large dose of unlabeled Bj
2
is given
intravenously (to saturate Bi
2
binding sites in tissues).
The radioactivity appearing in a 24-hour urine
specimen is determined. A normal person excretes
about one-third of the radioactive label in 24 hours,
while less than
8
% is found in the urine of a person
with pernicious anemia (a positive Schilling test). The
test is repeated several days later, this time including
IF with the vitamin B12. If the defect is lack of IF, the
uptake (and excretion) of the radioactive label will be
normal. Malabsorption of B
12
-IF complex in the
terminal ileum causes both parts of the Schilling test
to be abnormal (see below). The megaloblastic
anemia of folate deficiency is usually
indistinguishable from that of vitamin B
12
deficiency
except by the Schilling test.
3.
Transcobalantin II
(TCM). TCII is the principal
plasma-binding protein for cobalamins newly
absorbed from the intestine. Its normal plasma
concentration is 0.5-1.5 /xg/mL. The protein (M.W.
38,000) is synthesized in hepatocytes, fibroblasts,
macrophages, and perhaps enterocytes and other cells.
It is probably not a glycoprotein, making it distinct
from TCI and TCIII. It migrates with the /1-globulins
during serum protein electrophoresis and binds one
cobalamin per protein molecule.
TCII plays a major role in transport of cobalamins
to tissues. A receptor for the TCII-B
12
complex has
been tentatively identified
in vitro
on HeLa
cells,Ehrlich ascites cells,and human fibroblasts. The
bound complex is transferred into the cell, where the
cobalamin is released and the TCII is degraded in
lysosomes (Figure 38-18). An inborn error of vitamin
B
]2
metabolism has been attributed to a defect in
vitamin B
12
release from lysosomes. A congenital
