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

VIII. Ketogenesis and Ketosis
101
compound will reach peripheral tissues in an unoxidized
state.
Acetoacetyl-CoA thiolase and 3-hydroxybutyrate dehydrogenase
catalyze reactions that are at or near equilibrium
(i.e., the rates of these reactions are controlled by the concentrations
of the substrates and products). HMG-CoA synthase
and lyase catalyze reactions that are far removed from equilibrium,
and these enzymes may be subject to regulatory controls
other than the concentrations of substrates and products.
The synthase is considered to be the rate-limiting enzyme and
appears to be restricted almost exclusively to the liver ( Duee
et al ., 1994 ; McGarry and Foster, 1969 ; Valera et al ., 1994 ).
Physiological controls of the enzyme are not completely
understood; however, the concentration of enzyme molecules
in the mitochondria is increased by cAMP, so fasting and
diabetes increases it, and refeeding decreases it ( Serra et al .,
1993 ). In addition, succinyl-CoA inhibits the enzyme ( Quant
et al ., 1990 ). Glucagon usually decreases mitochondrial succinyl-CoA,
whereas an abundance of glucose or glucose precursors,
like propionate, increases it. A summary of ketogenesis
in the liver is depicted diagrammatically in Figure 4-1 .
Ketogenesis can occur from VFA and medium chain
fatty acids. Medium chain fatty acids are normally in quite
low concentration in the diet or in triacylglycerols of mammals
and, therefore, are not usually quantitatively important
in ketogenesis. Except in unusual circumstances, nonherbivores
do not absorb large quantities of VFA from the gastrointestinal
tract. Among the herbivores, the metabolism
of VFA has been studied most thoroughly in ruminants.
Propionate is the major gluconeogenic precursor and is not
an important precursor of ketones, and, in fact, propionate
inhibits ketogenesis in ruminant liver ( Faulkner and Pollock,
1991 ). The propionate inhibition probably is due to inhibition
of carnitine acyltransferase I in ruminant liver by methylmalonyl-CoA,
a metabolite of propionate ( Brindle et al .,
1985 ). Without active carnitine acyltransferase I, LCFA cannot
enter mitochondria and be oxidized to ketones.
Butyrate is converted to 3-hydroxybutyrate by the rumen
epithelium and will be discussed later. Acetate must be
covalently bound to CoA under the catalysis of acetokinase
before it can be catabolized further. Acetokinase is found in
the cytosol and mitochondria of most cells in most organs.
In ruminants, the liver has a relatively low concentration of
acetokinase, and most absorbed acetate passes through the
liver and is removed from the plasma by other tissues, particularly
heart, skeletal muscle, kidney, and mammary gland
( Bauman and Davis, 1975 ; Cook et al ., 1969 ).
2 . Ketogenesis by the Alimentary Tract
Butyrate, which is produced during fermentation of feedstuffs
in the rumen, is readily absorbed by the rumen wall
( Stevens, 1970 ). The rumen epithelial cells possess high
activities of butyryl-CoA synthetase, which can convert
butyrate to butyryl-CoA ( Cook et al ., 1969 ). By β-oxidation,
butyryl-CoA is converted to L-3-hydroxybutyryl-CoA,
which is oxidized to acetoacetyl-CoA followed by cleavage
of the CoA and reduction of the resulting acetoacetate
to 3-hydroxybutyrate (Emmanuel, 1980 ). Rumen epithelium
does have HMG-CoA synthase, HMG-CoA lyase
and 3-hydroxybutyrate dehydrogenase activities, although
in lesser concentration than in liver ( Baird et al ., 1970 ;
Emmanuel, 1980 ). It is possible that rumen epithelium
can cleave CoA from acetoacetyl-CoA directly because it
contains acetoacetyl-CoA deacylase ( Bush and Milligan,
1971 ). In addition, rumen epithelium possesses 3-ketoacid
CoA-transferase, an enzyme to be discussed later when
ketone oxidation is discussed ( Bush and Milligan, 1971 ).
This enzyme can catalyze the transfer of CoA from acetoacetyl-CoA
to succinate, thus liberating acetoacetate. This
latter route may be the predominant pathway in rumen epithelium
( Bush and Milligan, 1971 ).
3-Hydroxybutyrate appears in portal blood ( Katz and
Bergman, 1969 ; Stevens, 1970 ). At least 50% of absorbed
butyrate is oxidized to ketones in the rumen wall, and of
the butyrate that does appear in portal blood, nearly all of
it is removed on the first pass through the liver ( Bergman
et al ., 1965 ; Bergman and Wolfe, 1971 ; Fell and Weekes,
1975 ; Ramsey and Davis, 1965 ). Ruminal production of
3-hydroxybutyrate is probably the main reason why fed
ruminants normally have a higher plasma concentration of
this compound than fed nonruminants (see Table 4-3 ).
3 . Ketogenesis by Other Organs
It has been claimed that mammary gland may synthesize
appreciable ketones in ketotic dairy cows; however, the evidence
is weak. Arteriovenous concentration differences and
mammary blood flow have been used to estimate mammary
ketone production and uptake in dairy cows ( Kronfeld et al .,
1968 ). It was found that the mammary gland utilized small
quantities of acetoacetate and larger quantities of 3-hydroxybutyrate
in healthy cows, whereas the mammary gland of
ketotic cows produced large quantities of acetoacetate. The
increased uptake of 3-hydroxybutyrate by mammary in
ketotic cows equaled almost exactly mammary production
of acetoacetate. There was no significant difference in mammary
uptake of acetate between healthy and ketotic cows.
In yet another study on ketotic cows ( Schwalm et al .,
1969 ), arteriovenous concentration differences across the
mammary glands of acetoacetate and 3-hydroxybutyrate
were observed. A positive AV difference was noted for
3-hydroxybutyrate, which was almost equal in magnitude
to the negative AV difference noted for acetoacetate.
The foregoing results point toward mammary conversion
of 3-hydroxybutyrate to acetoacetate, which increases in
ketosis. This process cannot really be called ketogenesis;
perhaps ketoconversion would be the appropriate term.