N-Homocysteinyl-Proteins - Homocysteine in Protein Structure/Function and Human Disease - page 63

Table 5.5 Concentrations of Hcy carried on proteins in normal human blood (Recalculated from
the data of [79])
Protein Protein N-linked Hcy (nM) Protein S-linked Hcy (nM)

Hemoglobin a 27,800 3,300

Albumin b

2,800 7,300

-Globulin b γ 700 3,500

Fibrinogen b 14 19

<

Transferrin b

28 5

<

Antitrypsin b 24 7

HDL b

23 47

LDL b 17 9

a Concentrations in erythrocytes
b Plasma concentrations

Table 5.6 N-
homocysteinylation at Lys525
of albumin is increased in
human CBS deficiency (Data
from [213])

Genotype (n) N-Hcy-Lys525 peptide (%)

CBS +/+ (29)

1.02 1.21

CBS / (15) 3.99 2.60*

*P ¼ 0.0007

concentration of N-linked Hcy carried on hemoglobin in erythrocytes is 27.8 M

(Table 5.5 ), exceeding the concentration of plasma tHcy. Similar recalculation
shows that albumin and

μ

-globulin carry micromolar concentrations of N-linked
Hcy, whereas fibrinogen, transferrin, antitrypsin, HDL, and LDL carry nanomolar
concentrations of N-linked Hcy.
The value calculated for N-Hcy-albumin (0.36 mol%

γ

M in plasma,
Table 5.4 ) is higher than the value of N-Hcy-protein measured directly with normal
human plasma (0.51

¼ 2.8 μ

M) and closer to the mean values measured in plasma from
hyperhomocysteinemic CBS- and MTHFR-deficient patients on a Hcy-lowering
therapy (3.02

μ

M, respectively) [115]. The relatively high content of N-
linked Hcy in the commercial albumin preparations may possibly be due to
demethylation of methionine residues of albumin during its purification from
plasma. The values obtained for N-Hcy-albumin using a chemical assay [115] are
similar to the values obtained with the LC-MS assay, which shows that N-Hcy-
Lys525-albumin constitutes 1.0 % and 4.0 % of plasma total albumin in healthy
controls and CBS-deficient patients, respectively (Table 5.6 ) [213].
The value calculated for N-Hcy-fibrinogen (0.1 mol%

μ M and 4.4 μ

14 nM, Table 5.4 )is
consistent with direct assays of fibrinogen isolated from human subjects, which
show that in normal plasma, N-Hcy-fibrinogen levels are 35.8

¼

14.3 nM [115].
The N-linked Hcy carried on N-Hcy-fibrinogen constitutes about 10 % of total N-
linked Hcy present in normal plasma protein (0.35

0.13 to 0.51 0.11 μ M [79,

115, 300, 301].
The value calculated for N-Hcy-HDL (0.04 mol% 23 nM) is much smaller

than that obtained by a semiquantitative Western blot immunoassay of N-Hcy-
ApoAI in HDL isolated from human plasma (0.46-5.2

¼

M or 1.0-7.4 % of total
ApoAI) [303]. Possible reasons for the discrepancy are discussed in Sect. 5.5.4 of
this topic.

μ