CASEIN AND TYROSINE KINASE-ACTIVITIES OF DEVELOPING CHICK-EMBRYO LIVER - EFFECT OF TRIIODOTHYRONINE

Cytosol cAMP-independent quercetin-inhibited protein kinase of developing chick embryo liver was measured at three embryonic ages (days 12, 14 and 18) in the presence of casein and poly (Glu-Na, Tyr) 4:1 as substrates. In the early embryonic stages the tyrosine kinase was almost as active as casein kinase, but on day 18 the tyrosine phosphorylation was only 25% of the casein phosphorylation. Both kinase activities strongly increased by the end of embryonic development: 7-fold with casein and 2.6-fold with poly (Glu-Na, Tyr) 4:1. Triiodothyronine caused twice the stimulation of casein and tyrosine phosphorylation on day 12, but had no effect on day 18.