COBALTOUS CHLORIDE-MEDIATED INDUCTION OF RAT HEPATIC TRYPTOPHAN 2,3-DIOXYGENASE - IMPLICATIONS FOR THE USE OF THE ENZYME TO PROBE THE HEPATIC FREE HEME POOL
Hg. Liu et Ma. Correia, COBALTOUS CHLORIDE-MEDIATED INDUCTION OF RAT HEPATIC TRYPTOPHAN 2,3-DIOXYGENASE - IMPLICATIONS FOR THE USE OF THE ENZYME TO PROBE THE HEPATIC FREE HEME POOL, Cellular and molecular biology, 40(7), 1994, pp. 881-889
Subcutaneous administration of CoCl2, a well recognized inhibitor of h
epatic heme synthesis, to rats results in the functional stimulation o
f total (holo-+apo) tryptophan 2,3 dioxygenase (TDO), a hemoprotein an
d the key rate-limiting enzyme in the oxidative metabolism of tryptoph
an to formylkynurenine. Because basal holo-TDO activity is not altered
, TDO stimulation appears to be entirely due to CoCl2-mediated increas
e of its apoprotein. This apoTDO increase was blocked by conventional
inhibitors of protein synthesis (actinomycin D, cycloheximide), thereb
y revealing that such CoCl2-mediated apoprotein increase truly reflect
ed TDO induction. To determine whether the CoCl2-mediated TDO inductio
n involved the action of its natural physiological inducers (glucocort
icoids) or was due to direct CoCl2-regulation of the TDO gene, rats we
re adrenalectomized before CoCl2 administration. In adrenalectomized r
ats, CoCl2 failed to induce TDO, but induction was completely restored
on administration of the glucocorticoid hydrocortisone, but not of ad
renaline. These findings reveal that CoCl2-mediated TDO induction is i
ndirect and entails glucocorticoid participation. In addition, because
CoCl2 lowered the % heme saturation of TDO [=100 (holoTDO activity/to
tal (apo+holo)TDO activity] largely by increasing the apoTDO protein l
evels rather than by affecting the basal holo-TDO levels (as expected
from its inhibition of heme synthesis), these findings question the wi
dely accepted use of the relative intrahepatic % heme saturation of TD
O as a reporter of the hepatic ''free'' heme pool.