FISH NUTRITION
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TABLE XII. POLYUNSATURATED ACID FAMILIES
Palmitoleic, w7
16:lw7
18 : 107
Oleic, 09
18 : lw9
20 : lw9
Linoleic, w 6
18:2w6
18 : 3 w 6
20 : 3w0
20 : 406
22 : 4 w 6
Linolenic, w3
18 : 3w3
20 : 5w3
22 : 6w3
22 : 6w3
terminal and the nearest double bond to it along the carbon chain.
The number of carbon atoms between the terminal methyl and the
double bond nearest the terminal methyl is consequently fixed.
Moreover since a divinylmethane rhythm (-CH=CH-CH2-CH=
CH-) is maintained in desaturation reactions, the position of all double
bonds relative to the methyl end of the molecule is fixed. It follows
that the exact structure of an unsaturated fatty acid is given by
three numbers: (a) the number of carbon atoms in the chain, (b) the
number of double bonds and (c) the inclusive number of carbon atoms
from the terminal methyl to the carbon atom of the first double bond
from the methyl end. This last number is the omega (w) number.
For example linoleic acid has the structure
CHa-CH 2-CH 2-CH 2-CH 2 - 4 C H = CH)-CH 2 - ( C H = CH-)
CH2-CH,-CH2-CH2-CH2-CH2-CH2-COOH
and is termed 18: 2w6. Linolenic acid has the structure
CH3-CH2-CH=CH-CH2-CH
=CH-CH2-CH =CH-CHBCH~-CH2-CH2-CH2-CH2-CH,-COOH
and is termed 1 8 : 3w3.
By the omega system the unsaturated fatty acids belonging to each
series are designated by the distance of the first double bond from the
methyl terminal of the molecule. Thus one refers to " the w : 3 series "
or " the w : 6 series." Once a group is specified then the formulas of
all other members of that group, formed by chain elongation and
desaturation reactions, are known. There are four common familie5