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Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

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II. Long Chain Fatty Acids<br />

85<br />

Plasma<br />

LCFA<br />

by Glucagon<br />

by Insulin<br />

Acetoacetate<br />

3-Hydroxybutyrate<br />

Membrane<br />

LCFA<br />

ATP<br />

CoA<br />

Glycerol-3-P<br />

Cytosol<br />

&<br />

Microsomes<br />

LCFA-CoA<br />

Triglycerides<br />

–<br />

Malonyl-CoA<br />

Carnitine<br />

CoA<br />

LCFA-Carnitine<br />

Inner<br />

Mitochondrial<br />

Membrane<br />

CAT I<br />

CAT II<br />

3-Hydroxybutyrate<br />

NAD +<br />

NADH<br />

Acetoacetate<br />

Carnitine<br />

LCFA-Carnitine<br />

CoA<br />

Mitochondrial<br />

Matrix<br />

LCFA-CoA<br />

CoA<br />

b-Oxidation<br />

FAD + NAD +<br />

Acetyl-CoA<br />

Hydroxymethylglutaryl-CoA<br />

CoA<br />

FADH 2 + NADH<br />

Acetyl-CoA<br />

Acetoacetyl-CoA<br />

CoA<br />

FAD + NAD +<br />

Tricarboxylic<br />

Acid Cycle<br />

FADH 2 + NADH + CO 2<br />

FIGURE 4-2 Long chain fatty acid (LCFA) oxidation and ketogenesis in the liver. Abbreviations: CAT, carnitine<br />

acyltransferase.<br />

( Kilponen et al ., 1991 ). Polyunsaturated LCFA require an<br />

additional enzyme, 2,4-dienoyl-CoA reductase, because<br />

after enoyl-CoA isomerase acts, the new trans double bond<br />

will still have the second cis double bond in close proximity,<br />

which will prevent Δ 2 -enoyl-CoA hydratase from<br />

acting. 2,4-Dienoyl-CoA reductase effectively eliminates<br />

the second double bond by reducing it with NADPH ( Roe<br />

et al ., 1990 ).<br />

β-Oxidation in the mitochondria appears to be controlled<br />

mainly by substrate availability. The acetyl-CoA units can<br />

be oxidized in the citric acid cycle provided there is sufficient<br />

oxaloacetate to condense with them to form citrate.<br />

Alternatively, acetyl-CoA units can be recondensed to form<br />

ketones, which will occur when there is not sufficient oxaloacetate<br />

for citrate formation or when citrate synthase is<br />

inhibited by high levels <strong>of</strong> citrate. Although β-oxidation<br />

occurs mostly in mitochondria, the process occurs to a<br />

minor extent in peroxisomes as well ( Wanders et al ., 1992 ).<br />

Although the main catabolic route for LCFA is β-<br />

oxidation, there are two quantitatively minor alternatives.

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