Embryonic synthesis of myoglobin in vivo estimated by radioimmunoassay

Chick embryos in ovo incorporated radioactivity from lysine-U-¹⁴C into myoglobin, as measured by an immunoprecipitation technique. The most consistent results were obtained by injection of the precursor into the yolk sac fluid.Incorporation, or apparent myoblobin synthesis, occurred in cardiac and skeletal muscle but not in liver, although incorporation of amino acid into total soluble proteins was equivalent in all tissues studied. Synthesis was highest in cardiac muscle and appeared there first in younger embryos. Myoglobin synthesis was detectable in the heart of embryos as early as 6 days of age and rose with age thereafter. Myoglobin synthesis appeared later and at lower levels in skeletal muscle.In vitro at neutral pH, tissue extracts of liver and muscle possessed only slight properties of myoglobin degradation.Using nonradioactive precipitin techniques, sensitive to 5–10 μg/ml, myoglobin was detected in embryonic heart muscle by week 2 of life and rose in content thereafter. Two of 8 embryos had trace amounts in thigh muscle near the time of hatching, and no embryos possessed measurable amounts of myoglobin in liver tissue or in pectoral skeletal muscle. Adult birds possessed equivalent amounts of myoglobin in heart and thigh muscle while pectoral muscle and liver tissue had no detectable myoglobin content.