Food Lipids: Chemistry, Nutrition, and Biotechnology

first oxidation step. KCN-sensitive, palmitoylcarnitine (or palmitoyl CoA
plus L-carnitine)–dependent oxygen uptake by pea cotyledon mitochondria
has repeatedly been observed in the presence of malate [39,40]. The KCN
sensitivity of the reaction prevents peroxisomes from contaminating the
mitochondrial fraction.
3. Peroxisomes do not oxidize palmitoylcarntitine. NADH formation in an
NAD � -dependent reaction in mitochondria from avocado (Persea americana
L.) mesocarp or potato (Solanum tuberosum L.) tubers was greatly
stimulated by palmitoylcarnitine [41], while L-carnitine was ineffective.
However, Gerhardt [41] observes that palmitoylcarnitine has a disintegrating
effect on mitochondrial membranes at micromolar concentrations, resulting
in facilitated access of external substrates to intramitochondrial reaction
sites.
4. Activities of �-oxidation enzymes are normally detected in mitochondrial
fractions isolated from plant tissues on density gradients [13,19,42,43].
5. Enoyl-CoA hydratase and 3-hydroxyacyl-CoA dehydrogenase activities of
the peroxisomal �-oxidation system are carried out by one multifunctional
protein. In contrast, these activities belong to individual proteins in animal
mitochondria. A protein exhibiting only enoyl-CoA hydratase activity and
also differing from the peroxisomal multifunctional protein in other properties
was recently isolated from pea cotyledon mitochondria [44]. In addition,
antibodies raised against rat liver mitochondrial enoyl-CoA hydratase
gave a positive signal in Western blots of total pea mitochondrial
proteins; no signal is observed when blots of total pea peroxisomal proteins
are probed.
The results that argue against mitochondrial �-oxidation include the following:
1. When potato tuber mitochondria were provided with palmitoyl-L-carnitine
labeled at the C-1 position of the fatty acid moiety, no formation of
[ 14 C]acetyl CoA or labeled citrate was detected in the presence of malate,
nor was there any loss of radioactivity from the substrate [41].
2. The first oxidation step of mitochondrial �-oxidation in animal cells is
catalyzed by acyl-CoA dehydrogenase. All attempts to demonstrate acyl-
CoA dehydrogenase activity in higher plant mitochondria using long, medium,
and short chain acyl CoA’s as substrates have been unsuccessful
[13,19,22,27].
3. When the ratio of enoyl-CoA hydratase, 3-hydroxyacyl-CoA dehydrogenase,
or thiolase activity to the activity of the peroxisomal marker catalase
is calculated, no statistically significant difference is noted between the
ratios obtained for mitochondrial and peroxisomal fractions [12,45], indicating
that �-oxidation enzyme activities in mitochondrial fractions might
be attributed to contaminating peroxisomes rather than to mitochondrial
constituents.
It appears that direct demonstration of acetyl-CoA formation from palmitoylcarnitine
by mitochondria and/or of the first oxidation step of mitochondrial �-oxidation
are the primary lines of evidence needed to establish the existence of a mitochondrial
�-oxidation system in higher plant cells.
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.