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

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Chapter | 3 Carbohydrate Metabolism and Its Diseases
TABLE 3-10 Carbohydrate Balance Sheet
a
A. Cow’s daily glucose flux
1. In 12.5 kg milk: Carbohydrate carbon
610 g lactose 257 g C/day
462 g milk fat with 58 g glycerol 23 g C/day
Carbohydrate carbon in milk/day 280 g C/day
2. Daily glucose catabolism
Cow produced daily 3288 liters CO 2 1762 g C
Transfer quotient plasma glucose → CO 2 is 0.1
Thus glucose to CO 2 /day 176 g C/day
1 2 daily flux of glucose 456 g C/day
180
72 456 1140 g glucose/day
B. Cow’s glucose sources
Cow secreted daily in urine 34 g N,
indicating catabolism of 213 g protein 100 g C/day
Less C in urea
14 g C/day
Maximum available for glucose synthesis 96 g C/day
from protein
Glucose flow in milk and respiration
Thus glucose flow from nonprotein
sources
measured by the use of isotopically labeled glucose and
has been used in lactating cows. It has been estimated to be
1440 g/day (60 g/h) in cows and about 144 g/day (6 g/h) in
normal pregnant ewes just before term.
2 . Glucose Sources
456 g C/day
360 g C/day
180
72 360 900 g glucose daily must have been supplied from
a nonprotein source
a
From Kleiber (1959) .
The large amounts of indigestible carbohydrates ingested
by ruminants are fermented to volatile fatty acids by the
rumen microflora. Little, if any, of the digestible carbohydrates
(starch, glucose) in the diet escapes this fermentation,
so that glucose absorption from the digestive tract
accounts for virtually none of the daily glucose requirement
of ruminants. However, if any glucose escapes rumen
fermentation (e.g., in gastrointestinal disease), it is readily
absorbed.
An indirect source of blood glucose is ruminal lactic
acid. Lactic acid is a product of many fermentation reactions,
and ruminal lactate can be absorbed. The blood lactate
can be a source of blood glucose via the lactic acid cycle
( Fig. 3-4 ). However, the principal source of blood lactate is
the breakdown of muscle glycogen. Therefore, some of the
ruminant’s glucose requirement may be met by lactate, but
this is minimal because excess lactic acid in the rumen is
toxic.
The carbohydrate balance sheet ( Table 3-10 ) provides
the contribution of protein as a source of carbohydrate
for the lactating cow. Because glucose absorption in the
ruminant is minimal, the balance sheet also illustrates the
importance of an alternate nonprotein source of carbohydrate
carbon. These sources are the ruminal volatile fatty
acids. The principal products of rumen fermentation are the
volatile fatty acids, acetate, propionate, and butyrate. These
acids are absorbed across the rumen wall and are the major
source of nutriment for the ruminant. Various authors have
used a variety of techniques to estimate the amounts of
production and absorption of these acids. These fatty acids
are found in blood in approximately the following proportions:
acetate, 65; propionate, 20; and butyrate, 10. Further
details of fatty acid production and absorption by the ruminant
may be found in the chapter on lipid metabolism. In
general, carbon atoms of acetate, although they appear in
carbohydrate (blood glucose, milk lactose) through the
mechanism of rearrangement in the TCA cycle ( Fig. 3-9 ),
cannot theoretically contribute to the net synthesis of carbohydrate.
Thus, acetate is not a glucogenic compound.
The large amounts of acetate provided by rumen fermentation
are utilized for energy purposes and for the synthesis
of fat. A possible mechanism for the direct incorporation of
acetate into a glucose precursor is the so-called glyoxylate
pathway, which occurs in plants but not in animals.
Propionate, on the other hand, is a well-known precursor
of carbohydrate. The pathway leading to a net synthesis
of glucose from propionate is available via the reaction
propionate CO 2 → succinate
as shown in Figure 3-9 . According to the scheme, two
moles of propionate are required for the synthesis of a
mole of glucose, so 1g of propionate theoretically can provide
1.23g of glucose. The amounts of propionate available
from rumen fermentation can theoretically supply the glucose
requirements not accounted for by protein sources.
Butyrate, the third major fatty acid of rumen fermentation,
influences glucogenesis but does not contribute carbon
directly to glucose. Butyrate stimulates glucose production
by liver by increasing phosphorylases and gluconeogenesis.
The AcCoA derived from β -oxidation of butyrate
also activates pyruvate carboxylase, a key gluconeogenic
enzyme, which further promotes gluconeogenesis.
3 . Utilization of Glucose
The overall utilization of glucose by the ruminant has significant
differences from that of other animals. Acetate