Homocysteine-Thiolactone - Homocysteine in Protein Structure/Function and Human Disease - page 48

Table 3.11 Turnover of D -, D , L -, and L -Hcy-thiolactone in the mouse
Hcy-thiolactone
stereoisomer

HTL recovered in plasma,
% injected dose Plasma HTL t 0.5 , min

6.3 0.2 5.0 0.9

L -

88 3 5.7 0.3

D -

47 4 5.5 0.2

D , L -

Wild-type C57BL/6J mice are injected intraperitoneally with L -(n ¼ 3), or D , L -

stereoisomers (n ¼ 6) of Hcy-thiolactone at a dose of 150 nmol g 1 body weight. Plasma Hcy-
thiolactone (HTL) levels are assayed at time points up to 30-min postinjection. Plasma
concentrations at time zero (HTL 0 ) and half-lives (t 0.5 ¼ 0.69/k) are calculated from plasma
concentrations at time t (HTL t ) according to the equation [HTL t ] ¼ [HTL 0 ]·e k·t , where k is a
first-order rate constant. Hcy-thiolactone recovery calculations are based on assumptions that the
i.p.-injected Hcy-thiolactone, being mostly neutral (pK a ¼ 6.67), distributes uniformly throughout
the body and that blood constitutes 8 % of the mouse body weight (Data from [141])

6), D -(n ¼

Hcy-thiolactone-hydrolyzing enzyme(s), such as Blmh, is responsible for the
metabolism of Hcy-thiolactone during its transition from the intraperitoneal cavity
to the bloodstream in mice. In contrast, clearance of Hcy-thiolactone from the
bloodstream is non-stereospecific [141].
To study the physiological role of Blmh, a mouse model with inactivated Blmh
gene has been generated [272]. Blmh / mice are found to be more sensitive to
bleomycin toxicity than wild-type littermates, as expected. In addition, Blmh /
mice have somewhat lower body weight (by ~10 %), produce fewer pups in a litter
(60 % compared to wild type), and are prone to tail dermatitis [272].
The Blmh-null mouse model facilitated examination of the role Blmh in Hcy-
thiolactone metabolism and in the pathology caused by acute hyperhomocys-
teinemia. It was found that metabolic conversion of Hcy-thiolactone to Hcy is
impaired in Blmh-null mice. For example, Blmh / mice have elevated brain and
kidney Hcy-thiolactone levels and excrete more Hcy-thiolactone in urine, com-
pared with wild-type Blmh +/+ littermates [141]. These findings suggest that the
kidneys and the brain are major sources of increased urinary Hcy-thiolactone
excretion in Blmh / mice.
Furthermore, significantly more of intraperitoneally injected L -Hcy-thiolactone
is recovered in the plasma of Blmh / mice relative to wild-type animals (14.4 %
vs. 5.9 %, P < 0.0001). This means that, during its transit from the intraperitoneal

cavity to the bloodstream, L -Hcy-thiolactone is metabolized less efficiently in
Blmh / mice compared with Blmh +/+ animals. The i.p.-injected Hcy-thiolactone
is hydrolyzed to Hcy, which accumulates in the blood immediately after injection,
but is subsequently cleared with a half-life of about 30 min. However, less Hcy is
generated in Blmh / mice, compared with wild-type Blmh +/+ littermates [141].
Blmh / mice are also found to be more susceptible to Hcy-thiolactone neuro-
toxicity than wild-type littermates [141]. For example, when Blmh / and Blmh +/+
mice are i.p. injected with convulsant doses of L -Hcy-thiolactone (3,700 nmol g 1
body weight), seizures occur within 50 min. The incidence of seizures is signifi-
cantly increased in Blmh / mice compared with Blmh +/+ animals (93.8 %
vs. 29.5 %, P < 0.001) (Table 3.10 ). Seizure latency (i.e., time to first seizure) is