Developmental changes of glycogen phosphorylase b isozymes in rat tissues.

The isozyme distribution of glycogen phosphorylase b was studied in various fetal, neonatal, and adult rat tissues by means of discontinuous polyacrylamide gel electrophoresis in the presence of glycogen in the separating gel. The brain-type isozyme BB being the predominant form in fetal rat tissues is replaced during maturation either partially (brain and kidney) or completely (skeletal muscle and liver) by the isozymes MM or LL, respectively. In the various organs, the developmental formation of the adult phosphorylase isozyme pattern does not proceed simultaneously. The definitive isozyme pattern is expressed in heart muscle at the 19th day of gestation. The isozyme transitions in liver and skeletal muscle are finished about two days after birth, while at this time in the brain just the muscle-type subunit M appears and forms the hybrid MB. The isozyme distribution of the maturated rat brain (BB, MB, MM) is not expressed before the 9th day of postnatal life.