Why is the 18:3 w-3 fatty acid (9,12,15-octadecatrienoic acid)
named alpha-linolenic acid, while the 18:3 w-6 fatty acid (6,9,12-
octadecatrienoic acid) named gamma-linolenic acid ?
It's just convention – there's no systematic reason.
Looking at the structure of alpha-linolenic acid (usually just called
linolenic acid), you will see that the unsaturation (the three C=C
double bonds) are all arranged towards the methyl end of the
molecule. The first double bond (starting from the carbon at the
methyl end, C18, which is these days somewhat perversely known
as the omega-carbon atom) is situated between carbon 16 and
carbon 15. Let us call this the alpha double bond.
So the next position (working away from the methyl end) in which
you could have a double bond would be between carbons 14 and
13. The desaturase capable of this dehydrogenation does occur in
nature, but very rarely (although the reaction can be carried out in
the test tube) and hence we do not normally come across beta-
linolenic acid.
Moving further along the molecule from C18, the next (third)
position for a C=C bond to exist in this chain is between
carbons 13 and 12, and when this compound is formed it is called
gamma-linolenic acid. If you look at the structure, you'll see that
the de-saturation (and hence the flexibility) is symmetrically
arranged about the centre of the molecule, whereas in the case of
alpha-linolenic acid it is biased towards the C18 end. Just to make
this more interesting for everyone else, the effect of feeding gamma-
linolenic acid (in the form of evening primrose oil or sunflower oil) is
possibly to produce prostacyclins that are less pro-inflammatory
than those derived from alpha-linolenic acid, and this is probably
related to this structural difference.
Saying that these double bonds are "alpha" and "gamma" is based
on a decision taken by workers in the field many years ago (I'm not
sure when, but it pre-dates the alpha / omega system of naming
fatty acids structures or the understanding of the w-3, w-6
systems). It is not logical, but it is still universally used.
Best wishes,
Nick Miller,
London
> Dear collegues!
> Can anyone explain me the nomenclatur of long-chain polyunsaturated fatty
> acids concerning the difference between alpha- and gamma. For example:
> alpha-linolenic acid C18:3(9,12,15)
> gamma-linolenic acid C18:3
> Alpha-and gamma is not due to position and number of double binding, but
> is it due to cis and trans configuration (alpha = all cis, beta = all
> trans and gamma cis,trans)?
>
> Dr. Elisabeth Haschke-Becher
> Laboratoire Central de Chimie Clinique
> Centre HospitalierUniversitaire Vaudois
> Bugnon 46
> CH - 1011 Lausanne
> Tel: 0041/21-3144129
> Fax.: 0041/21-3144288
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