Studies on the thyroid gland component inhibiting LATS

As described by others, the soluble (that is, 100,000 × g. supernatant) fraction of homogenates of porcine and human thyroid gland contained long-acting thyroid stimulator (LATS) inhibitor; it was indistinguishable from thyroglobulin on gel filtration in Sepharose 2B. However, inhibiting activity remained after hydrolysis with trypsin. Chromatography of the hydrolysate on Sephadex G-100 provided two LATS-inhibitory fractions, one of which was excluded by the gel; antiserum directed against the original gland extract contained predominantly antithyroglobulin but neither inhibitory fraction interacted with the antiserum. From these data it is clear that the inhibitor of LATS need not be equivalent in size to 19S thyroglobulin, nor have the antigenic determinants of that molecule. Although no 5′-nucleotidase activity was found in the soluble fraction of liver-homogenates, over 50 per cent of that enzyme was recovered in the soluble fraction of thyroid homogenates. It may be, therefore, that material normally sited in the cell membrane is solubilized during homogenization of the thyroid. The relevance of this finding to the subcellular site of origin of the LATS-inhibitor is discussed.