AJP - Regulatory, Integrative and Comparative Physiology, Vol 258, Issue 3 756-R762, Copyright © 1990 by American Physiological Society
Mitochondrial and peroxisomal fatty acid oxidation in elasmobranchs
C. D. Moyes, L. T. Buck and P. W. Hochachka
Department of Zoology, University of British Columbia, Vancouver, Canada.
In heart and red muscle of dogfish (Squalus acanthias), the maximal
activities of the fatty acid catabolizing enzyme carnitine
palmitoyltransferase (CPT) are less than 5% the rate in the same tissues of
teleosts (carp, Cyprinus carpio; trout, Salmo gairdneri). CPT activities in
these tissues of hagfish (Eptatretus stouti) are approximately 10% the rate
in teleosts. However, the maximal activities of the beta-oxidation enzyme
beta-hydroxyacyl-CoA dehydrogenase (HOAD) in dogfish red muscle and heart
are similar to these tissues in the other species. This paradox prompted a
more detailed study on the capacity of mitochondria from dogfish cardiac
and red skeletal muscles to utilize fatty acids, possibly by a
CPT-independent pathway. Free fatty acids were not oxidized by mitochondria
from red muscle (hexanoate, octanoate, decanoate, and palmitate) or from
heart (octanoate, palmitate). Neither hyposmotic incubation nor addition of
5 mM ATP could stimulate oxidation of octanoate or palmitate in either
preparation, suggesting that these tissues have little capacity to oxidize
fatty acids by a carnitine-independent pathway. Palmitoyl carnitine
oxidation was detectable at very low rates in these mitochondria only with
hyposmotic incubation. Octanoyl carnitine was oxidized at greater rates
than palmitoyl carnitine, 10% the rate of pyruvate in both tissues,
suggesting that medium-chain fatty acids could be physiologically relevant
fuels in elasmobranchs if available to heart and red muscle. One potential
source of medium-chain fatty acids is hepatic peroxisomal beta-oxidation,
which occurs in dogfish liver at maximal activities similar to carp and
trout liver. However, based on relative rates of oxidation, it is likely
that dogfish heart and red muscle metabolism are fueled primarily by
carbohydrate and ketone bodies.
