M. Erecinska et al., LIMITATION OF GLYCOLYSIS BY HEXOKINASE IN RAT-BRAIN SYNAPTOSOMES DURING INTENSE ION PUMPING, Brain research, 726(1-2), 1996, pp. 153-159
Incubation of rat brain synaptosomes under conditions of either increa
sed energy utilization (addition of Na+ channel opener, veratridine, o
r ionophores, monensin and nigericin) or inhibition of oxidative phosp
horylation (addition of rotenone), or a combination thereof, decreased
[ATP], increased [ADP] and stimulated glycolysis. The rates of lactat
e generation were linear over a 15-min interval in the presence of rot
enone alone but decreased in the other two conditions. During the firs
t 5 min, the amount of lactate formed with veratridine, monensin or ni
gericin was as high or higher than with rotenone, but it was lower in
the last 10 min. With a combination of one of the stimulators of ion m
ovements and rotenone the rate of glycolysis was always markedly lower
than with each compound added singly. The stimulated rates of lactate
formation correlated positively with the synaptosomal content of [ATP
]. After 15 min, [ATP] was 0.9-1.0 nmol/mg with rotenone, 0.5-0.9 nmol
/mg with veratridine (or ionophores), and < 0.3 nmol/mg with a combina
tion of the two. Under the conditions used, calcium did not affect gly
colytic activity directly. The Lineweaver-Burk plot of the rate of lac
tate formation against [ATP] yielded a straight line with a K-m for AT
P of about 0.1 mM, which is very similar to the K-m for this nucleotid
e of brain hexokinase bound to mitochondria. In C6 cells glycolytic ra
te measured with a combination of an ionophore and rotenone was higher
than with each of these compounds added singly while [ATP] never decl
ined below about 9 nmol/mg prot. It is concluded that in synaptosomes,
the high rate of energy utilization required for intense ion movement
s decreases [ATP] to a level that limits hexokinase activity kinetical
ly. This may contribute to a reduction in the rate of glycolysis and h
ence energy production in brain hypoxia and ischemia.