THE PHOSPHOLIPIDS OF SHEEP THYROID By N. FREINKEL*

There is much evidence for the importance of phosphorus in the metabolism and function of the thyroid gland. Kocher (1902) first described an inverse relationship between the contents of iodine and phosphorus in this tissue. More recently, Dempsey & Singer (1946) have presented histochemical evidence for marked fluctuations in the activities of acid and alkaline phosphatases and of hexose phosphatases in the gland during periods of varying thyroid activity, and Borell & Holmgren (1949) have demonstrated that the uptake of radioactive phosphorus by the thyroid in vivo is increased by pituitary thyrotropin. With preparations in vitro, Morton & Schwartz (1953) have reported that pituitary thyrotropin selectively increases the incorporation of P2p into the phospholipids of ox-thyroid slices, and Freinkel & Ingbar (1955a) have shown that the first step in the synthesis of thyroid hormone, namely the accumulation of Iion, is dependent upon energy derived from oxidative phosphorylation. Despite these data, detailed inquiry into the composition and metabolic pathways of thyroid tissue has not hitherto been made. The present paper deals with phosphoglyceride metabolism. The phospholipids and related compounds which occur in sheep thyroid have been characterized by recently developed chromatographic techniques (Dawson, 1954, 1955a). Moreover, these methods have been used to study the incorporation in vitro of both ["2P]phosphate and [14C]glycerol into the individual phosphoglycerides and their metabolic intermediates in sheepthyroid slices. The studies have yielded information about the metabolic pathways of the phospholipids, and the active turnover of these components suggests that the phospholipids may