The differential binding of lactate dehydrogenase (LDH) isozymes to ribonucleoprotein particles

An attempt was made to explore the possibility that in higher organisms gene expression might be controlled at the cytoplasmic sites of protein synthesis by interfering with the release of certain proteins from their templates. The existence of such mechanisms would be indicated, for instance, by the presence of tissue-foreign proteins in the ribosome fraction, such as tissue-atypical LDH isozymes. Ribosomes and postmicrosomes were prepared from guinea pig liver and kidney in 0.88 M sucrose +10⁻⁴M MgCl₂ solution and analyzed by starch gel electrophoresis for the isozyme patterns of particle-associated LDH. Ribosomes isolated from both organs were found to be associated with LDH. The enzyme could not be removed from the particles by repeated resuspension in sucrose medium, but treatment of the particles with agents, which cause a disintegration of their structure, as well as with salt solutions of high ionic strength released LDH in nonsedimentable form. The isozyme patterns of particle-associated LDH were different from those of the total homogenate. A correlation was found between the electrophoretic mobility of a given isozyme and the amounts of it which were recovered from the ribosome fraction. The results indicated that basic isozymes were preferentially bound to the ribonucleoprotein particles during fractionation in solutions of low ionic strength and did not represent genuine ribosomal enzyme proteins.