Fructose metabolism in the udders of dairy cows

¹ Department of Animal Husbandry, University of California, Davis, California

The metabolism of fructose by mammary tissue was studied by infusing U-C¹⁴ fructose into the one quarter of a lactating cow's udder. Comparison of specific activities of milk products isolated from the injected and uninjected quarters showed that fructose, unlike glucose, was not converted to lactose by mammary tissue. However, fructose was oxidized in the udder to metabolic intermediates which lead to the synthesis of milk protein, butterfat, citrate and CO₂. It appeared that little fructose C¹⁴ was absorbed by mammary tissue. These results were discussed with regard to the known metabolic pathways of fructose metabolism. It is postulated that an active fructokinase enzyme system functions in mammary tissue and further, that the principal direction of metabolic reactions in the udder favors catabolism of hexose rather than its synthesis.