Inhibition of cytochrome c oxidase activity during prolonged
hypoxia.
Chandel, Navdeep, G. Scott Budinger, Robert A. Kemp, and Paul T.
Schumacker.
Section of Pulmonary and Critical Care, Department of Medicine,
MC6026, The University of Chicago, 5841 S. Maryland Ave., Chicago,
Illinois 60637
APStracts 2:0010L, 1995.
During acute (< 30 min) hypoxia, cellular respiration is independent
of the O2 concentration as long as PO2 remains above a critical value
(5-10 torr). Similarly, State 3 respiration by isolated mitochondria
is independent of PO2 above a critical tension of 2-4 torr. However,
rat hepatocytes demonstrate a reversible suppression of respiration
and an increase in NAD(P)H concentration during prolonged (2-24 hrs),
but not acute hypoxia (Am J Physiol, 265:L395-L402, 1993). This study
tested whether respiration is similarly inhibited in isolated
mitochondria exposed to low PO2 for prolonged periods, and whether
cytochrome c oxidase participates in this response. Coupled rat liver
mitochondria were incubated under low oxygen conditions (PO2 < 2
torr) for 2 hours. State 3 respiration after reoxygenation to PO2 =
20 torr was then compared with the value obtained subsequently at 100
torr. Using succinate and adenosine diphosphate (ADP) as substrates,
State 3 respiration at 20 torr was 61.0 +/- 8.4% of the subsequent
value at 100 torr (P < 0.05). By contrast, control mitochondria
reoxygenated to 100 torr first and 20 torr subsequently showed no
significant difference at the two O2 tensions (P = N.S.). When
N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) was used as substrate
to supply electrons directly to cytochrome c, respiration at 20 torr
in mitochondria incubated at low PO2 for 2 hrs was 50.7 +/- 6.0% of the
value measured subsequently at 100 torr. To further test the role of
cytochrome c oxidase in this response, isolated bovine heart enzyme
was incubated at PO2 = 20 torr for 4 hr. In the presence of excess
substrate, O2 consumption by the enzyme at PO2 = 20 torr decreased
progressively, reaching a turnover rate of 23.5 +/- 1.3 sec-1 compared
with 40.7 +/- 0.9 sec-1 during reoxygenation to 100 torr (P < 0.001)
after 4 hr. We conclude that electron transport in rat hepatocyte
mitochondria is reversibly inhibited after prolonged exposure to low
PO2. This inhibition appears to be mediated by a regulatory effect of
molecular oxygen on the catalytic behavior of cytochrome c oxidase.
These findings provide a mechanistic explanation for the reversible
decrease in respiration by intact hepatocytes during prolonged
exposure to hypoxia.
Received 9 June 1994; accepted in final form 27 January 1995.
APS Manuscript Number L165-4.
Article publication pending Am. J. Physiol. (Lung Cell. Mol.
Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 24 February 1995.