Page 87 - Small Animal Clinical Nutrition 5th Edition

Macronutrients 87

gradually declines as the amino acids from the meal are used. aids therapy for chronic renal failure (Chapter 37).
VetBooks.ir Because protein turnover and obligatory nitrogen losses con- nucleotides, ATP and GTP, RNA and DNA) may also create
Catabolism of purines (adenine and guanine from
tinue to occur, muscles must convert from net uptake to net
nitrogen disposal problems. Uric acid is the end product of
release of amino acids as fasting becomes dominant. Although
glycogen and lipids are the preferred short-term and long- purine catabolism in people, nonhuman primates, birds and
term energy stores, protein catabolism can supply carbon Dalmatian dogs; all other mammals produce allantoin. Uric acid
skeletons needed to maintain glucose pools and supply the and allantoin are excreted in urine and feces. Accumulation of
body with energy (ATP). Protein degradation continues until uric acid in serum can lead to gout (uric acid crystals in joints)
the next meal, after which protein synthesis exceeds protein and uric acid uroliths. Hyperuricemia can be treated with drugs
breakdown. (allopurinol) and with dietary intervention. Reducing total pro-
The carbon skeletons of catabolized amino acids can be tein intake and reducing or eliminating dietary ingredients con-
completely oxidized to carbon dioxide or converted by the taining high levels of purines (organ meats, glandular meats, fish
liver to glucose or ketone bodies. Thus, amino acids can be such as mackerel and sardines) have been successful in some
glucogenic or ketogenic. Glucogenic amino acids can be dogs with urolithiasis (Chapter 39).
converted to glucose through pyruvate or any of the TCA Protein-losing enteropathy (PLE) and protein-losing
cycle intermediates. Ketogenic amino acids are those that nephropathy (PLN) are two syndromes in which excessive
can be converted to acetyl-CoA or ketone bodies but cannot amounts of protein are lost from the body. PLE is a group of
be metabolized to glucose. Some catabolized amino acids diseases characterized by excessive loss of serum protein into
yield two different carbon skeletons, one ketogenic and the the intestinal tract. Causes of PLE include lymphatic disorders
other glucogenic (Table 5-11). (lymphangiectasia, neoplasia, congestive heart failure) and dis-
eases associated with increased mucosal permeability or mucos-
Protein Excretion al ulceration (intestinal neoplasia,granulomatous enteritis,lym-
Amino acid catabolism produces waste nitrogen, if it is not phocytic-plasmacytic enteritis, intestinal parasitism, etc.).
used for purine or pyrimidine synthesis. Catabolism of amino PLN is characterized by excessive loss of serum protein into
acids typically leads to formation of ammonia, which is toxic urine. Causes of PLN include glomerulonephritis and amyloi-
to body cells. Ammonia must be converted to a less toxic form dosis that develop secondary to chronic inflammatory condi-
that can be transported in blood and excreted. Ammonia for- tions such as neoplasia, heartworm disease, ehrlichiosis, sys-
mation occurs in all tissues, but is especially prevalent in the temic mycosis, brucellosis, osteomyelitis and immune-mediat-
liver and kidneys during gluconeogenesis. More than 90% of ed disease.
the nitrogen resulting from protein degradation is converted to
urea in the liver and kidneys and then excreted in urine. A Analyses of Protein
smaller fraction of nitrogen is lost as ammonia, creatinine and Chemical Analyses of Protein
nitrate. Hepatic and renal detoxification of ammonia via the Protein concentration in foods (crude protein content) is typi-
urea cycle is a critical function.The amino acids ornithine, cit- cally calculated from the nitrogen content by the Kjeldahl
rulline and arginine are key intermediates. Other amino acids method or Dumas combustion procedure. The Kjeldahl
(e.g., glutamate, aspartate, alanine and the amide group of glu- method involves digesting the food sample with sulfuric acid,
tamine) carry excess amino groups from other tissues to the which converts all nitrogen in amino acids to ammonia. The
liver and kidneys for conversion to urea. Catabolism of other ammonia is distilled, collected and quantified using a colori-
nitrogen-containing metabolites also contributes to the waste metric titration procedure. The Dumas combustion procedure
nitrogen pool. requires no hazardous materials and involves measuring the
nitrogen gas after combustion and total destruction of the sam-
CLINICAL PROBLEMS ASSOCIATED WITH ple.This method has been adopted as an acceptable method for
NITROGEN DISPOSAL measuring total nitrogen in animal foods (Etheridge et al,
Hyperammonemia may result if any of the enzymes involved 1998). When using either method, two assumptions are made
in the urea cycle are lacking or have impaired activity. Part of the in calculating crude protein from nitrogen. First, that all nitro-
therapy for any disease (e.g., liver disease) in which urea cycle gen in the food sample is present as protein and second, that all
function is reduced is to decrease plasma concentrations of protein contains 16% nitrogen. The crude protein content of
ammonia. Reducing total protein intake, reducing intake of the food is then calculated: crude protein in food (g/kg) = g
nonessential amino acids and substituting keto analogues of nitrogen in food/kg x 6.25.
some amino acids may help reduce the ammonia load. Other Neither assumption is always true. Not all nitrogen in food is
measures (e.g., dialysis, transfusion, administration of drugs and present as protein; some nitrogen is present as free amino acids,
other compounds that trap nitrogenous metabolites) can be used amides, glycosides, alkaloids and ammonium salts. Other nitro-
to prevent an ammonia encephalopathy crisis (Chapter 68). gen is complexed with lipids. Different protein ingredients in
Reduced renal function may also impair nitrogen disposal food also have different nitrogen contents and, therefore, differ-
(i.e., urea may not be excreted efficiently). Reducing waste ent conversion factors (Table 5-12).The use of an average con-
products of protein catabolism through dietary manipulation version factor (i.e., 6.25) for all proteins in food simplifies cal-