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

IV. Water-Soluble Vitamins
719
Enzyme
(CH 3 ) 4 HB
NH 2
H O
C
CH 2 OPO 3
O
3
O
(CH 3 ) 4
Enzyme
HN +
C
H
CH 2 OPO 3
3
H 3 C
+
N
H
H
H 3 C
(CH 3 ) 4
ENZYME
+
N
H
H
H or R
H H
: O
H
C
:NH 2
COOH
NH 2
Transaminations
Elimination Reactions
H
R Group eliminations and additions
H
C COOH
Decarboxylations
HC
NH +
O
CH 2 OPO 3
3
H 3 C
FIGURE 23-18 Pyridoxine. Vitamin B 6 is a collective term for pyridoxine (-CH 2 OH), pyridoxal (-CHO), and pyridoxamine (-CH 2 -NH 2 ). The active
form pyridoxal (B 6 cofactor form) is phosphorylated. The type of reactions carried out by vitamin B 6 fall into three general categories. These are mainly
reactions that apply to the metabolism and interconversion of amino acids. The most common of these is the transaminase reaction. Transaminations
are essential to the interconversion of amino acids to corresponding α -keto acids. The transamination mechanism also applies for reactions important to
producing racemic amino acid mixtures. Examples of α , β -elimination reactions are the conversion of serine to pyruvate or the conversion of homocysteine
plus serine to cystathionine. The second most common reaction involves electron withdrawal from the α -carbon and carboxylic acid group carbon.
This facilitates decarboxylation reactions. A third type of reaction involves electron withdrawal from the α , β -carbons of amino acids. This sets the stage
for hydride condensations or aldol reactions (e.g., the formation of α -aminolevulinic acid), the first step in heme biosynthesis.
+
N
H
H
The acid proton in this instance is derived from the phosphate
group of pyridoxal 5 -phosphate. Before the elucidation of
this important function, it was speculated that the association
of vitamin B 6 with glycogen phosphorylase was primarily
some type of storage mechanism. Indeed, muscle is a
good source of vitamin B 6 , but its presence in muscle relates
mostly to its role as a catalyst in glycogen hydrolysis.
b . Metabolism and Requirements
The requirement of vitamin B 6 by animals is positively
related to their intake of protein and amino acids; however,
vitamin B 6 deficiency is rarely seen in animals as most
diets provide adequate amounts. Normally, B 6 is needed in
amounts that range from 2 to 6 mg/kg diet. Ruminants and
many herbivores meet a substantial part of their vitamin B 6
requirement from intestinal microbes. Administration of
the tuberculostatic drug isoniazid induces a metabolic deficiency
of vitamin B 6 .
As might be expected, the most important signs of B 6
deficiency relate to the inability to carry out normal amino
acid metabolism. Neurological signs occur as a result of
the inability to synthesize important biogenic amines from
amino acid precursors and anemia results from decreased
heme synthesis. Under experimental conditions, some animals
may show signs of oxaluria with long-term deficiencies
in vitamin B 6 ( Committee on Animal Nutrition, 2001b ).
c . Determination of Vitamin B 6 Status
A number of tests have been used as an index of vitamin
B 6 status. These include measurement of the activities of
enzymes that require pyridoxal phosphate such as kynureninase
and aminotransferases. However, the most sensitive
methods involve the measurement of pyridoxal and
pyridoxal phosphate in blood plasma. The relative ratio of
these two forms and their response to dietary intake of pyridoxine
depends on the species of animal.
3 . Pantothenic Acid
a . Introduction
Pantothenic acid was first recognized as a growth factor
for yeast and lactic acid bacteria in the 1930s. Later,