Biochemical changes in progressive muscular dystrophy. VII. Studies on the biosynthesis of protein and RNA in various cellular fractions of the muscle of normal and dystrophic mice

Abstract

Protein nitrogen content, RNA concentration, and in vivo incorporation of L-[U-14C]leucine into protein and of [2-14C]uridine into RNA of homogenate and various fractions of muscle of normal and dystrophic mice were measured at various stages of the disease. Protein nitrogen content was always lower in dystrophic than in normal muscle, and this became more pronounced with the progress of the disease. Most of the decrease was due to loss of proteins from the myofibrils. RNA content increased in the homogenate, nuclei–myofibrils, supernatant, and microsomes of dystrophic muscle. In the mitochondria of dystrophic muscle, no change was noted compared to controls. The ratio of RNA content to protein in the homogenate, nuclei–myofibrils, supernatant, and microsomes was also greater in dystrophic muscle. It was not changed in dystrophic muscle mitochondria. Incorporation of L-[U-14C]leucine into proteins of dystrophic muscle homogenate and various fractions also increased to variable degrees over that in the controls. It was further observed that mitochondrial and microsomal protein incorporate L-[U-14C]leucine in dystrophic muscle at an increased rate but the disappearance of 14C was even greater, compared to controls.In vivo incorporation of [2-14C]uridine into RNA of dystrophic muscle increased at 30 days', remained the same at 60 days', and declined at 90 days' duration of the disease. Similar results were also obtained in the nuclei–myofibrillar fraction of dystrophic muscle. In all other fractions an increase was noted in incorporation in dystrophic muscle. The incorporation of [2-14C]uridine into RNA in supernatant and microsomes was higher in dystrophic muscle but the disappearance of 14C was greater, compared to controls. It is quite evident in the microsomal fraction at 90 days, where no change in the incorporation is noted in normal and dystrophic animals.