Chloramphenicol/thiamphenicol and cycloheximide as tools for the measurement of mitochondrial protein synthesis in vitro during organogenesis of rat embryos.

1. A test system was developed to allow the measurement of protein synthesis in vitro in mitochondria from tissues which were accessible only in small quantities. The subcellular fractions which could be isolated are not purely mitochondrial but contain other particles as well, mainly microsomal, which are also active in protein synthesis. The following differences between mitochondrial and microsomal protein synthesis in vitro were used to measure selectively the mitochondrial portion in cell fractions sedimenting between 600 and 10000 X g: selective inhibition of mitochondrial protein synthesis by chloramphenicol/thiamphenicol selective inhibition of microsomal protein synthesis by cycloheximide kinetics of amino acid incorporation a medium favoring mitochondrial protein synthesis Activity of mitochondrial protein synthesis was based on measurements of cytochrome oxidase, a mitochondrial marker enzyme. 2. The technique developed was used for the evaluation of mitochondrial protein synthesis in mammalian embryonic tissues. It may equally well be applied to other tissues available in small amounts and in cases where the isolation of highly purified mitochondrial fractions is met with difficulty. 3. Comparing the rate of 14C-phenylalanine incorporation into mitochondrial protein from rat embryos at different stages of gestation, it was found that mitochondria from 11=day-old rat embryos exhibit an approximately 30-fold higher capacity for protein synthesis than those of day 13-16. On day 12 the capacity is 6 times higher than on the following days.