Back in Protons (53) a formula I wrote down how to work out the F:N ratio of (even chain) fatty acids with varying double bonds:
where n is the length of the carbon chain and db is the number of double bonds.
Oleic C18 is 18-1-1 divided by 36-1, ie 16/35 = 0.457
Linoleic 18-1-2 divided by 36-1, ie 15/35 = 0.423
This is fine up to C18 but C20 and above are targeted to peroxisomes rather than mitochondria so the need for an F:N ratio fades. Peroxisomes have their own signalling systems but research on them is in its infancy.
Anyhoo, Tucker mentioned off blog that during the multistep processing of double bonds there is a step which consumes NADPH. This will have to be re-reduced from the resultant NADP+ by the Krebs Cycle where NADH producing steps have iso enzymes capable of generating NADPH instead of NADH. That reduces the NADH supply to the electron transport chain by 1 NADH per double bond requiring NADPH, so complicates the formula.
The formula ends up as:
F/N = (n-1-db)/(2n-1-db)
It makes a relatively small change to the ratio as the denominator is a much larger number than the numerator.
Oleic acid, originally 0.457 becomes
18-1-1 divided by 36-1-1, ie 16/34 = 0.471
and linoleic acid, originally 0.423 becomes
18-1-2 divided by 36-1-2, ie 15/33 = 0.455
The latter is interesting as it moves linoleic acid upwards towards MUFA and the saturates because the denominator drops.
The value for caprylic (shortest saturate in common consumption) is 0.467 and with the new LA now at 0.455, they are getting closer. Also caprylic is now at lower F:N ratio than oleic. I just wonder if this is part of the explanation of the coconut based diets used by Surwit to induce obesity with LA still limited to 4% of calories...
Thanks to Tucker for the NADPH requirement insight.
Peter
