GLYOXYSOME BIOGENESIS VIA THE ENDOPLASMIC RETICULUM IN CASTOR BEAN ENDOSPERM?

Malate dehydrogenase isoenzymes: cellular locations and role in the flow of metabolites between the cytoplasm and cell organelles

Malate dehydrogenases belong to the most active enzymes in glyoxysomes, mitochondria, peroxisomes, chloroplasts and the cytosol. In this review, the properties and the role of the isoenzymes in different compartments of the cell are compared, with emphasis on molecular biological aspects. Structure and function of malate dehydrogenase isoenzymes from plants, mammalian cells and ascomycetes (yeast, Neurospora) are considered. Significant information on evolutionary aspects and characterisation of functional domains of the enzymes emanates from bacterial malate and lactate dehydrogenases modified by protein engineering. The review endeavours to give up-to-date information on the biogenesis and intracellular targeting of malate dehydrogenase isoenzymes as well as enzymes cooperating with them in the flow of metabolites of a given pathway and organelle.

Characterization of membrane-bound electron transport enzymes from castor bean glyoxysomes and endoplasmic reticulum

Membranes purified from castor bean endosperm glyoxysomes by washing with sodium carbonate exhibited integral NADH:ferricyanide and NADH:cytochrome c reductase activities. The enzyme activities could not be attributed to contamination by other endomembranes. Purified endoplasmic reticulum membranes also contained the redox activities; and marker enzyme analysis indicated minimum cross contamination between glyoxysomal and endoplasmic reticulum fractions. The glyoxysomal redox activities were optimally solubilized at detergent to protein ratios (weight to weight) of 10 (Triton X-100), 50 (3-[3-cholamidopropyl)-dimethylammonio]-1-propanesulfonate), and 100 (octylglucoside). Detergent in excess of the solubilization optimum was stimulatory to NADH:ferricyanide reductase and inhibitory to NADH:cytochrome c reductase. Endoplasmic reticulum redox activity solubilization profiles were similar to those obtained for glyoxysomal enzymes using Triton X-100. Purification of the glyoxysomal and endoplasmic reticulum NADH:ferricyanide reductases was accomplished using dye-ligand affinity chromatography on Cibacron blue 3GA agarose. Sodium dodecyl sulfate-polyacrylamide gel electrophoretic analysis of NADH:ferricyanide reductase preparations purified by rate-zonal density gradient centrifugation, affinity chromatography, and nondenaturing electrophoresis of detergent-solubilized glyoxysomal and endoplasmic reticulum membranes consistently displayed 32- and 33-kDa silver-stained polypeptide bands, respectively.

Microbody transition in greening watermelon cotyledons Double immunocytochemical labeling of isocitrate lyase and hydroxypyruvate reductase

Microbody transition during the greening of watermelon cotyledons (Citrullus vulgaris Schrad.) was studied by double immunocytochemical labeling of the glyoxysomal marker enzyme isocitrate lyase and the peroxisomal marker enzyme hydroxypyruvate reductase. In order to analyze the immunocytochemistry, developmental stages representing the glyoxysomal, microbodytransition and peroxisomal stages were chosen, taking into account the time course of enzyme activity and the amounts of the respective antigens. It was shown that during microbody transition, between 83 and 91% of all the tested microbodies contained isocitrate lyase as well as hydroxypyruvate reductase, which was significantly higher than in the glyoxysomal and peroxisomal stages of development. Comprehensive controls precluded labeling artifacts. Our results support the one-population hypothesis first proposed by Trelease et al. (1971, Plant Physiol. 48, 461-465).

Double-membraned vesicles and their possible role in the ontogeny of microbodies of Basidiobolus haptosporus

A concomitant but transient occurrence of large numbers of double-membraned vesicles with immature, culturally induced microbodies was demonstrated in thin sections of Basidiobolus haptosporus soon after transfer of the fungus to a defined medium containing xanthine or its catabolites as the sole source of nitrogen. Double-membraned vesicles were rapidly formed as derivatives of endoplasmic cisternae later to occur free in the cytoplasm, often as the most predominant cytoplasmic inclusion. Densitometric measurements revealed that heavy metal-binding cytoplasmic constituents were concentrated within the vesicular lumen. Most of the double-membraned vesicles appeared to undergo degeneration; they were rare to absent in thin sections of older cells. At times, double-membraned vesicles were seen connected to newly formed microbodies in a manner suggesting ontogenic relationships. The outer vesicular membrane was continuous with the single limiting membrane of the microbody via a short tubule. We interpret these observations as empirical ultrastructural evidence to suggest that some double-membraned vesicles become specialized and function as precursor inclusions in the biogenesis of new microbody populations committed to purine salvage.

Synthesis in vitro of precursor-type carnitine acetyltransferase with messenger RNA from Candida tropicalis

Carnitine acetyltransferase was synthesized in vitro in the mRNA-dependent reticulocyte system with mRNA from alkane-grown or propionate-grown cells of Candida tropicalis. The protein synthesized in vitro was isolated by immunoprecipitation with antibody against peroxisomal or mitochondrial carnitine acetyltransferase and was compared with peroxisomal carnitine acetyltransferase (Mr of subunits, 64 000 and 57 000) and the mitochondrial enzyme (Mr of subunits, 64 000 and 52 000) of C. tropicalis by electrophoresis in the presence of sodium dodecyl sulfate. Nascent carnitine acetyltransferase prepared in vitro showed a hetero-oligomeric property, like the peroxisomal and mitochondrial enzymes isolated from C. tropicalis. The molecular weights of the subunits of nascent carnitine acetyltransferase were estimated to be 71 000 and 57 000, indicating the existence of the precursor form of the enzyme. By sucrose density gradient centrifugation of total mRNA, these two subunit proteins were shown to be synthesized with respective mRNAs of different sizes. The same precursor-type of carnitine acetyltransferase was obtained with the mRNAs from the alkane-grown cells and the propionate-grown cells. The results obtained suggest that a common precursor will be post-translationally modified to form the peroxisomal and mitochondrial enzymes.