Effects of amino acid analogs on growth and catheptic activity of chick embryo explants

Protein nitrogen content and catheptic activities were determined in homogenates of dissected organ primordia from chick embryos grown in ovo. The effects of amino acid analogs on protein nitrogen content and catheptic activities were investigated in chick embryos grown as explants. The dependence of the measured catheptic activity on substrate concentration was found to follow the Michaelis-Menten equation, and the time course of catheptic activity was linear for a 9-hour incubation period. During incubation in ovo from 40 to 50 hours, the specific catheptic activity decreased about equally in the brain, the neural tube, the axial mesoderm, and the heart. During incubation in ovo from 50 to 85 hours, the specific catheptic activity of the heart showed a three-fold increase, the specific catheptic activity of the neural tube and axial mesoderm doubled, and the specific activity of the brain showed no significant increase. In tests with chick embryos explanted at the 11–13 somite stage, leucine analogs were found to reduce the net increase in protein nitrogen of the brain, neural tube, and axial mesoderm to values as low as 20% (1.02 × 10 −3 M γ-bromoallylglycine) of the protein nitrogen increase in the controls. The total catheptic activity in the primordia was only insignificantly affected by the presence of leucine analogs. Fluorophenylalanine not only reduced the increase in protein nitrogen, but also depressed significantly the total catheptic activity in the examined organ primordia. The significance of catheptic activity in embryonic tissue and the differential effects of amino acid analogs on total protein content and catheptic activity are discussed.