260 G. PopjAK 



be synthesized in the gland from carbohydrate sources 

 (Graham, Houchin, Peterson and Turner, 1938). To explain 

 the occurrence of the short -chain acids several speculative 

 views have been put forward. It was suggested for instance 

 by Reineke, Stoneciphcr and Turner (1941) that if part of 

 the milk fat is synthesized in the gland from carbohydrate, 

 the short-chain acids might be the part so formed. The most 

 fully developed theory, however, that of Hilditch, assigns no 

 synthetic role to the mammary gland, but insists that the 

 volatile acids are derived from blood glycerides, especially 

 their oleic acid component (Hilditch, 1947) by an a>-type of 

 oxidation (Achaya and Hilditch, 1950). 



In view of the central role of acetate in fat metabolism as 

 revealed by the work of Bloch and Rittenberg (Bloch, 1947, 

 1948), Folley suggested in a chapter written in 1945 for the 

 new edition of Marshall's Physiology of Reproduction (which 

 unfortunately is still not published) that the short-chain 

 acids in milk might also originate by synthesis from acetate. 



Folley and French (1948, 1950) have found that tissue 

 slices made from the lactating mammae of ruminants utilized 

 acetate (but not glucose) with a respiratory quotient (R.Q.) 

 greater than one. On the other hand, slices from the glands 

 of non-ruminants (rat, rabbit) showed a R.Q. greater than 

 one only in the presence of glucose, or glucose +acetate, 

 but not with acetate alone as substrate. They interpreted 

 their results as indicating the preferential use of acetate rather 

 than glucose for milk fat synthesis in ruminants. 



In the course of studies on foetal fat metabolism Miss 

 Beeckmans and I investigated the fat in the mammary gland 

 of non-lactating pregnant rabbits. We were able to show 

 after giving DgO and ^^C-labelled acetate to the animals that 

 active fat synthesis occurred in the gland (Popjak and 

 Beeckmans, 1949, 1950). There was a particularly high 

 incorporation of ^^C from acetate into the glyceride fatty 

 acids and it was subsequently shown that this was largely 

 due to the uptake of acetate into the short chain volatile 

 fatty acids (Popjak, Folley and French, 1949; Popjak and 



