Mode of formation of milk fatty acids from acetate in the goat.

In a previous paper (PopjAk, French & Folley, 1951) some results of an experiment carried out on a lactating goat with the aid of CH314CO2Na were described. It was found that, in the origin of milk fat, synthesis within the udder from acetate played an outstanding part. This conclusion was based on a comparison of the specific activities of four crude fatty-acid fractions obtained from milk glycerides with those of the plasma fatty acids after a single injection of CH314CO2Na. During the first 12 hr. of the experiment the steam-volatile acids insoluble in water (consisting mainly of C8-C12 acids) had the highest specific activities, the next in order being the water-soluble steam-volatile acids (C4-C8 acids). These were followed by the specific activity of the non-volatile solid and liquid acids, the latter having the lowest specific activities. From 12 hr. onwards, however, the specific activity-time curve of the non-volatile solid acids gradually crossed over the specific activity-time curves of the two steam-volatile fractions so that by the 23rd hour of the experiment the specific activities of the four fractions were in the following order: non-volatile solid acids> waterinsoluble steam-volatile acids> water-soluble steamvolatile acids> non-volatile liquid acids. This same order was maintained until the end ofthe experiment (48 hr.) (cf. Popjak et al. 1951, Fig. 3). The specific activity-time curves of all four fractions, after reaching their maxima 3-4 hr. after the injection of labelled acetate, decayed smoothly with an average half-life of about 4 hr. The similarity of the half-lives suggested that the different specific activities ofthe fractions were in somewayconnected with the biochemical mechanism of fatty-acid synthesis rather than with any other factor (e.g. differences in turnover time, dilution with nonisotopic blood fat). Since the specific activities of these mixtures of fatty-acid samples bore no simple relationship to the average chain length of their component acids, it was pointed out that resolution into individual acids was required in order to interpret more fully the role of acetate in milk-fat synthesis. In this paper the results of complete fractionations of the fatty-acid samples obtained in the previously * Member of the staff of the National Institute for Research in Dairying, University of Reading, at present working at the National Institute for Medical Research. described experiment are reported together with the chemical degradations of acetic, butyric and caproic acids, in which the position of the '4C-label in individual carbon atoms of the fatty-acid chain has been established. It would have been a gargantuan task to resolve into individual acids the four crude fatty-acid fractions from twenty-two milk samples. If the specific activities ofthe four fractions hadmaintained the same order relative to one another throughout the experiment, it would have been justifiable to pool the samples from the entire experiment for the isolation of the individual acids. However, because of the change in the relative position of the specific activity-time curves at 12 hr. after the injection, it was decided to divide the acids into two main groups, (1) those obtained up to 12 hr. and (2) those between 12 and 48 hr. of the experiment. Thus during the first period the fatty acids had their highest specific activities and during the second period the specific activities of all the fractions were already declining. In addition to these two main parts of the experiment, some results obtained on two small samples of the water-soluble volatile acids, one representing 0-7 hr. and the other approximately the entire experimental period, are also presented. It isnow apparent that, although acetate is amajor source of carbon for milk fatty-acid synthesis, the results strongly suggest that another precursor, which we believe to be a C4 compound, derived from the blood, must also be involved. As a result of the participation of this second precursor of low isotope content, it has been possible to show by chemical degradations that the synthesis of fatty acids must occur by the stepwise elongation of a shorter acid at the carboxyl end with a C2 compound and not by the simultaneous condensation of C2 units. Some of these results have been published in a preliminary form (French, Hunter, Martin & Popjak, 1951).