Fractionation of Carbon Isotopes During Carbohydrate Fermentation in Ruminants

Abstract

In the forestomaches of ruminants the carbohydrates of the diet are transformed by microbiotic fermentation via pyruvate into short chain-fatty acids (acetate, propionate, and with less quantity butyrate). Since carbohydrates, i.e. glucose, cannot penetrate the rumen epithelium, metabolically needed glucose must be resynthesized intermediary mainly from propionate by gluconeogenesis.

Lactose in milk of lactating cows is formed up to 98% from glucose of that origin 1]. Glucose absorption is possible in the small intestine, but contrary to monogastric animals, this is without quantitative relevance.

Therefore, an experimentally found higher abundance of ¹³C in lactose of about 3‰, compared with milk-fat 2], is not explainable only by passing on the well known isotopic differences between carbohydrates and fat in the diet. According to the conclusion of De NIRO and EPSTEIN 3], who identified the decarboxylation of pyruvate to be the relevant step of ¹³C depletion in fat in cultures of Escheria coli, we assume an isotopic shift between propionate and acetate produced by the ruminal microorganisms using pyruvate as the last common precursor of both fatty acids.

In vitro-investigation of ¹³C/¹²C - ratios in CO₂ produced during the decarboxylation of pyruvate in fresh rumen-fluid, confirmed the responsibility of that enzyme-reaction for isotope fractionation also in the multiple-step-system of ruminal carbohydrate conversion.