Spontaneous in vitro hybridization of LDH homopolymers in the undenatured state

Induction of both rat and mouse lactate dehydrogenase in hybrids between inducible rat glioma and uninducible mouse neuroblastoma cells

The specific activity of lactate dehydrogenase (LDH; EC 1.1.1.27) is induced two-fold by l-norepinephrine (NE) in C6TK- rat glioma cells, but not in NA mouse neuroblastoma cells or various other nonglial cells. Previous reports have shown that the induction is mediated by cyclic AMP (cAMP) and possibly protein phosphorylation, and that it requires RNA and protein synthesis. To study the block to LDH induction in nonglial cells, we hybridized C6TK- cells with NA cells and isolated a hybrid clone in which LDH is inducible by NE. Mouse and rat LDH from hybrid cells were separated by electrophoresis and quantitated by two independent methods, and it was found that mouse and rat LDH were induced equally when cells were exposed to NE. The results suggest that inducibility of LDH is not determined by a cis-acting control at the gene level, but rather by the presence or absence of an earlier component in the cAMP-mediated induction system, and that the induction system acts indiscriminately on all active LDH gene copies in the cell.

Evidence for the lack of feedback regulation of gene activity and for the absence of subunit exchange between lactate dehydrogenase tetramers in vivo

Frog and rat lactate dehydrogenase (LDH, L-lactate:NAD+ oxidoreductase, EC 1.1.1.27) isozymes, highly purfied by affinity chromatography, were injected into newly fertilized from (Rana pipiens) eggs to study the effect of the injected isozymes on the expression of the LDH genes of the developing frog embryo. From these experiments three conclusions can be drawn: (i) homologous and heterologous LDH, even when present in more than twice the normal amount of LDH, does not play any role in the switching on or off of the frog LDH genes or in regulating the level of gene activity. No evidence was found in support of any feedback regulation of LDH synthesis by the LDH molecule itself. (ii) Injected rat LDH is very stable in the frog embryo, and the contrary to what can be demonstrated in vitro, all the isozymes show the same stability. No evidence was found for selective degradation of any of the injected five isozymes. (iii) The fact that the injected isozymes disappear 12 days after the injection without forming hybrid molecules between themselves or with the stored or newly synthesized frog LDH is evidence in favor of the hypothesis that the tetramer is the degradative unit of LDH, not the monomer as others have postulated. The LDH tetramers do not dissociate the recombine under the physiological conditions present in frog cells.

Enzyme histochemistry as a link between biochemistry and morphology

The presented paper describes the role of enzyme histochemistry in cell biological investigations. In the first chapter a general discussion has been given about enzyme histochemistry as a connecting link between biochemistry and morphology. The methods available for determination of enzymes in a particular cell or cell compartment have been reviewed. In this respect the characteristics of enzyme histochemistry have been discussed. Furthermore, attention has been paid to the possibilities and limitations of enzyme histochemistry. In chapter two a comparison has been made between histochemically judged and biochemically determined enzyme activities. Some fundamental differences between the biochemical and the histochemical approach in cell biological investigations are dealt with. To correlate histochemically and biochemically determined enzyme activities, a description has been given of the application of histochemical methods on isolated fractions and sucrose-ficoll gradients of these fractions. Several experimental results are described concerning the question whether a relation exists between histochemically and biochemically determined activities of respectively alkaline phosphatase, glucose-6-phosphatase, 5'-nucleotidase and 3ss-hydroxysteroid dehydrogenase. From these results the conclusion could be drawn that in general a good correlation exists between histochemically judged activity per volume (area X thickness) and biochemically determined activity per gram tissue. In chapter three the role of enzymes as markers of cellular particles and as parameters of metabolic pathways is described. Histochemical methods are available for most marker enzymes. Only activities of key enzymes can be regarded as parameters of metabolic pathways. The distribution in sucrose-ficoll gradients of enzymes, regarded as markers of mitochondria, lysosomes, endoplasmic reticulum and plasma membranes has been given. The changes occur ing under different experimental conditions for a number of marker enzymes in rat liver are described. Attention has been given to the contibution of enzyme histochemistry in the study of the heterogeneity of mitochondria, the dual localization of some (lysosomal) enzymes, the complexity of the microsomal fraction, the function of the Golgi apparatus and the heterogeneity and function of plasma membranes. Based on these results and on literature findings the possible role of some marker enzymes in cell metabolism has been discussed. In chapter four problems coherent with species and sex differences in enzyme activities are described. The interpretation of histochemical and biochemical results in view of these differences is discussed. Enzymes characteristic for a given cell type -3ss-hydroxysteroid dehydrogenase in steroid producing cells, ATP-ase in liver plasma membrane surrounding the bile canaliculi - do show less variations between species and sexes than enzymes not directly involved in specialized functions...