Formation of Fatty Acids 



277 



activities of the four fractions decreased with increasing mean chain- 

 length. It is noteworthy that the specific activities of the udder fat 

 fractions from the udder half perfused with acetate are around ten times 

 as high as those of the corresponding milk fat fractions. This is prob- 

 ably due to the fact that the milk formed during the two-hour perfusion 

 period was diluted by milk which was already present in the udder and 

 which could not be removed completely before the perfusion, even with 

 the aid of oxytocin, and due also to some extent to the rapid turnover 

 of fat in the udder tissues. The fractions isolated from the right half 

 of the udder, which was perfused with labelled bicarbonate, had very 

 low specific activities which could only be measured by using an 

 internal gas counter. These trivial activities indicate that in the udder 

 tissue the incorporation of CO 2 into the milk fat is neghgible. 



We also have figures for lactose, not included in this Table. Lactose 

 from the udder half perfused with acetate showed appreciable radio- 

 activity, but the lactose secreted by the bicarbonate side was almost 

 totally inactive. Kleiber at Davis, California, after injection of 

 i*C-labelled bicarbonate into the lactating cow, found active lactose in 

 the milk — "hot lactose" as he called it. It would thus appear that 

 in Kleiber's experiment COg had been incorporated into the glycogen in 

 the liver, giving rise to labelled glucose which was transported from the 

 liver to the udder and there used for the synthesis of lactose. 



My second point concerns the relation of glycerol to milk fat synthesis. 

 To begin with, I should like to refer to Table II, taken from Balmain 

 and Folley (1951), which shows the effect of insulin on the in vitro 

 synthesis of fat by lactating rat mammary gland slices. In these 

 experiments the slices were incubated in a mixture of acetate and 

 glucose as substrate. In the presence of insulin we regularly obtain with 

 lactating rat mammary tissue an R.Q. of about 2, whilst without it the 



Table II 



Comparison of the Effects of Insulin and Glycerol on the Respiration 

 AND Glucose and Acetate Utilization of Lactating Rat Mammary 

 Gland Slices in Glucose (0-3 per cent) and Sodium Acetate (0-02 M) 



(from Balmain and Folley, 1951) 



Respiratory data and Qacid measured by method of Dickens and Simer (1931); 

 medium: Krebs-bicarbonate-Rlnger, pH 7 • 4; gas phase: 95 per cent Og, 5 per 

 cent CO2; temperature, 37°; duration of experiments, 2 hr.; crystalline insulin 

 (Novo) added to final concentration of 1 i.u. (0 05 mg.)/ml.; final concentra- 

 tion of glycerol, 0-2 per cent. Ten rats (13-15 days of lactation) used for 

 respiration measurements; glucose determinations carried out on 7 of these. 



