The oxidation of linoleate and other long-chain fatty acids in rat and sheep liver mitochondria : a thesis presented in partial fulfilment of the requirement for the degree of Doctor of Philosophy in Biochemistry at Massey University, New Zealand
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1986
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Massey University
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Abstract
Sheep liver mitochondria oxidised palmitate, oleate and
linoleate at slower rates than did rat liver mitochondria.
Rat liver mitochondria oxidised linoleate at 1.2 to
1.7 times the rate observed with palmitates as the substrate.
However, sheep liver mitochondria oxidised linoleate at 0.74
to 0.84 the rate observed when palmitate was the substrate.
The biochemical basis of this difference is not understood.
The reaction catalysed by the enzyme carnitine
acyltransferase I is believed to be an important regulatory
step in the oxidation of long-chain fatty acids and is known
to be competitively inhibited by malonyl-CoA . Both rat and
sheep liver mitochondria were able to form acyl carnitine when palmitoyl-CoA and linoleate, coupled with anacyl-CoA generating system, were the acyl substrates.
Malonyl-CoA was very effective in inhibiting the CAT I
reaction in sheep liver mitochondria. When linoleate,
coupled with an acyl-CoA generating system, was the
substrate for CAT I, 1uM malonyl-CoA was found to inhibit
the reaction by 90%. However, when the same substrate was
assayed in rat liver mitochondria the inhibition was much
less, 22 uM malonyl-CoA leading to only 50% inhibition of
the CAT I enzyme. When palmitoyl-CoA was used as
a substrate for the enzyme CAT I, little difference was seen
between rat and sheep liver mitochondria in the extent of
inhibition observed over the concentration range of 1 to
5 uM malonyl-CoA.
These experiments indicate that sheep liver
mitochondria could oxidise palmitate rather than linoleate
at low levels of malonyl-CoA as one might expect in vivo.
In contrast, in rat liver mitochondria, linoleate would be
oxidised faster than palmitate at all concentrations of
malonyl-CoA investigated.
It is suggested that this system may be an important
means whereby sheep are able to conserve linoleate by
preventing its oxidation.
In addition the mitochondrial glycerol3-phosphate
acyltransferase reaction was investigated with both sheep
and rat liver mitochondria. With linoleate and anacyl-CoA
generating system, rat liver preparations esterified
1.5 nmoles min/mg protein whereas sheep liver mitochondria
esterified less than one tenth of this. It was concluded
esterification of linoleate to glycerol 3-phosphate is not
an important mechanism of conserving linoleate in sheep
liver mitochondria.
Esterification of palmitate to glycerol 3-phosphate was
studied using palmitoyl-CoA as the acyl donor. At maximal
rates of esterification it was observed that rat liver
mitochondria esterified palmitoyl-CoA at 2 nmo les/mi n/mg
whereas sheep mi tochondr i a e s ter i f i ed 0 . 8 nmo l e s /mi n/mg .
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Keywords
Sheep liver mitochondria, Rat liver mitochondria, Mitochondria, Fatty acids