Page 132 - Small Animal Clinical Nutrition 5th Edition

Minerals and Vitamins 133

rylated forms, whereas it occurs mostly as free thiamin in plants.
Table 6-6. Thiaminase activity in fish products.*
VetBooks.ir low concentrations. The richest sources are whole grains, yeast Food Thiaminase activity**
Thiamin is widely distributed in foods, but is mostly present at
0
Marlin
and liver (especially pork liver). Meat products may also supply
significant amounts of thiamin. Up to 90% of thiamin in natu- Yellowfin tuna 265
265
Red snapper
ral ingredients may be lost as a result of processing (Morris and Skipjack tuna 1,000
Rogers, 1994). Therefore, thiamin supplementation is common Dolphin (mahi mahi) 120
Ladyfish 35
in pet foods. Thiamin hydrochloride and thiamin mononitrate Clam 2,640
are the most commonly used supplements in commercial foods *Adapted from Hilker DM, Peter OF. Anti-thiamin activity in
for dogs and cats. Hawaii fish. Journal of Nutrition 1966; 89: 419-421.
**mg thiamin destroyed/100 g fish/hour.
Riboflavin
Riboflavin, or vitamin B belongs to the class of isoalloxazines.
2
Riboflavin has a planar structure and has limited solubility in impaired reproduction, neurologic changes and anorexia
water. This property has clinical significance because it is diffi- (NRC, 1985; Street and Cowgill, 1939; Street et al, 1941).
cult to deliver massive doses of the vitamin via intravenous Measurement of erythrocyte glutathione reductase activity is
solutions. Riboflavin is heat stabile, but sensitive to light, and commonly used to evaluate riboflavin status in people and
acidic and alkaline conditions. animals. Table 6-5 lists riboflavin blood values for dogs and
cats. Most commercial pet foods are supplemented with syn-
FUNCTION thetic riboflavin. Toxicity has not been reported to occur in
Riboflavin is the precursor to a group of enzymatic cofactors dogs and cats.
called flavins. Flavins linked to protein are called flavoproteins.
The two major coenzymes derived from riboflavin are flavin SOURCES
mononucleotide (FMN) and flavin adenine dinucleotide (FAD). Riboflavin is widely distributed in foods, primarily bound to
Flavins are used as coenzymes in about 50 enzymes in mammals. proteins as FMN and FAD. Rich sources include dairy prod-
Flavins participate in intermediary energy metabolism and func- ucts, organ meats (e.g., liver, heart, kidney), muscle meats, eggs,
tion mainly in oxidoreductase types of reactions (Figure 6-4). green plants and yeast. Cereal grains are poor sources of vita-
min B . The supplemental form for addition to foods is usual-
2
METABOLISM ly riboflavin.
Most riboflavin found in food sources is in the form of free
coenzyme derivatives that are not readily absorbed unless Niacin
hydrolyzed, and covalently bound riboflavin that is not well Niacin is the generic term used to describe compounds that
used. The free flavin compounds are hydrolyzed before they exhibit biologic activity of nicotinamide. Two major forms of
are absorbed in the upper GI tract. A specialized transport niacin are nicotinic acid and nicotinamide.
system that is saturable and sodium dependent is necessary
for absorption of flavins. After absorption, about 50% of the FUNCTION
riboflavin in blood is bound to albumin and the other half to Nicotinic acid and nicotinamide are substituted pyridine ring
globulins (Brody, 1994a; Rivlin, 1996). Tissues requiring structures (pyridine 3-carboxylic acid and nicotinic acid amide).
riboflavin convert it to FMN by phosphorylation catalyzed by Niacin must be converted to either nicotinamide-adenine din-
flavokinase and subsequently to FAD catalyzed by FAD syn- ucleotide (NADH) or nicotinamide-adenine dinucleotide
thase. Excess riboflavin in the body is eliminated largely as phosphate (NADPH) to participate in enzymatic reactions or
riboflavin via the kidneys. protein modification.
Niacin, in its cofactor form, is essential to several physiolog-
REQUIREMENTS ic reactions: 1) oxidoreductive reactions, 2) nonredox reactions,
The AAFCO (2007) recomended allowance for riboflavin is 3) cleavage of β-N-glycosidic bonds with transfer of ADP-
2.2 mg/kg DM for dogs and 4 mg/kg DM for cats for all ribose to proteins (post-translational modification) and 4) for-
lifestages. The NRC (2006) recommended allowance for mation of cyclic ADP-ribose (mobilizes intracellular calcium).
riboflavin for dogs is 5.25 mg/kg DM for growth and mainte- Oxidoreductive reactions are the primary function, but the
nance and 5.3 mg/kg DM for gestation and lactation. For cats, others are significant in proper cell function. Generally,
the NRC (2006) recommended allowance for riboflavin is 4.0 NAD/NADH is involved in catabolic reactions and transfers
mg/kg DM for all lifestages. Table 6-5 lists AAFCO and NRC the reducing power (electrons) acquired from intermediary
allowances for dogs and cats. metabolites to the electron transport chain to ultimately produce
adenosine triphosphate. Alternatively, NADP/NADPH is gen-
DEFICIENCY AND TOXICITY erally involved in biosynthetic reactions that transfer reducing
Deficiency of riboflavin in dogs and cats is uncommon but power (electrons) to macromolecules such as fat, protein and
may manifest as dermatitis, erythema, weight loss, cataracts, carbohydrate.