Trans fatty acid, from hydrogenation mechanism

Toxicity, chemicals and, 25-26 Trans fatty acid, from hydrogenation of fats, 232-233 from vegetable oils, 1063 Transamination, 1165-1168 mechanism of, 1167... 

A few fatty acids with trans double bonds (trans fatty acids) occur naturally, but the major source of trans fats comes from partial hydrogenation of vegetable oils in, for example, the preparation of margarine. The same catalysts that catalyze the hydrogenation of the double bonds in a triacylglycerol also catalyze their stereoisomerization. The mechanism for conversion of a cis to a trans double bond follows directly from the mechanism of catalytic hydrogenation (see Section 6.1) once one realizes that all of the steps in the mechanism are reversible. 

Since hydrogenation is not carried to completion there is considerable scope for isomerization to occur. The mechanism by which this takes place has been described by Allen and Kiess (1955). They indicate that selective conditions tend to favor isomer formation. Both positional and geometric isomers are formed. In positional isomers, double bonds have wandered from their original position along the fatty acid carbon chain in geometric isomers the position of groups attached to carbon atoms have changed relative to each other in space from the natural cis to trans. Recently the nutritional properties of isomeric fatty acids have been questioned. Applewhite (1981) in a literature review has concluded that this concern is not substantiated by the available data. 

But where do all the other isomers come from Desaturation should not occur in the reducing environment of the rumen and thus could not explain the array of isomers observed as desaturation of rra r-monoenes. On the other hand, hydrogenation is efficiently accomplished when rumen pH is appropriate for the organisms involved in the isomerization and hydrogenation of PUFA (16). Several of the organisms appear to have specific selectivity for certain structures of the PUFA on which they act. Different microbes could have different preferences for PUFA isomers, rumen pH, or nutrients but there is insufficient information at this time to clearly describe the rumen flora in a way that would explain the plethora of fatty acid isomers observed in the rumen and milk of dairy cows. The following discussion may provide insights into possible mechanisms to explain the presence of the myriad of trans, cis, and CLA fatty acids in rumen and milk fat. 

---