Subcellular distribution of histone-degrading enzyme activities from rat liver

An alkaline thiol proteinase in the liver mitochondria of bullfrog, Rana catesbeiana

The mitoplasts were prepared from bullfrog (Rana catesbeiana) liver mitochondria by treatment with digitonin and were then separated into the matrix and inner membrane fractions. The matrix fraction thus obtained was free of lysosomal contaminations and exhibited a distinct proteinase activity. pH dependency of the matrix proteinase activity measured in the presence and absence of iodoacetamide revealed that the matrix contained at least two kinds of proteinase, a major alkaline thiol proteinase having an optimal pH at 8.5 and a minor neutral proteinase having an optimal pH at 7.5. The major matrix proteinase activity was strongly inhibited by leupeptin, chymostatin, antipain and E64-C, an inhibitor of Ca2+-dependent thiol proteinase, while it was scarcely affected by diethylpyrocarbonate. The activity was also inhibited by DTNB and p-chloromercuribenzoate. Addition of hydrocarbon compounds such as ethylene glycol, glycerol, Triton X-100 and poly (ethylene glycol) to the reaction mixture was found to decrease the matrix proteinase activity. Neither cytochrome c nor glutamate dehydrogenase was hydrolyzed when subjected to the matrix proteinase activity in vitro. On the other hand, cytochrome c oxidase was effectively hydrolyzed, and the enzyme associated with the mitochondrial innermembrane fragments was partially hydrolyzed by the major matrix proteinase activity.

Study on the localization of proteases of mitochondrial origin

A marked proteolytic activity against casein can be demonstrated in rat liver mitochondria. The proteases degrading casein appear distributed between a sedimentable fraction (Po) and a soluble extract (So). Part of the soluble fraction activity, which may be recovered in the mitochondrial intermembrane space, results from a contamination by lysosomal proteases and can be eliminated by previously washing the mitochondria with digitonin. The pre-exposure to digitonin causes an enhancement of the caseinolytic activity associated with the membrane fragments, proving that this activity is not due to lysosomal enzymes. When rats have been injected in vivo with the compound 48/80 which, by degranulating the mast cells prevents contamination of the mitochondrial preparations by mast cell proteases, the membrane fraction (Po) retains a caseinolytic activity of the order of 80 per cent of the control preparations. A similar value of activity is observed in the membranes of brain mitochondria, isolated by a method which removes the rare mast cells they may contain. This shows that the greater part of the caseinolytic activity associated with the rat liver membranes does not originate from mast cell granules. Liver mitochondria pre-exposed to digitonin to eliminate lysosomal contaminants, have been subfractionated into matrix, intermembrane space, inner and outer membrane. Each of the fractions exhibits a caseinolytic activity, but the largest part is localized in the inner compartments of mitochondria: the matrix and the inner membrane. The optimal pH and the sensitivity to inhibitors of the proteases in the different compartments indicate that we are dealing with distinct enzymes.

Phosphorylation of chromatin proteins in cerebral anoxia and ischemia

Phosphorylation of nuclear protein was investigated in cerebral anoxia up to 30 min with rabbit brain and in cerebral ischemia up to 6 h with gerbil brain in vitro. Isolated nuclei were incubated in the presence of [gamma-32P]ATP and were then fractionated into the NaCl-soluble, HCl-soluble, and phenol-soluble protein fraction. Each protein fraction was further separated by gel electrophoresis, and profiles of 32P incorporation were evaluated in these pathophysiological conditions. 32P incorporation of the acidic phenol-soluble nonhistone chromatin protein became significantly suppressed in cerebral anoxia after 15 min, and there were decreases of 32P incorporation in protein with high molecular weight and increase in protein with low molecular weight on gel electrophoresis. With gerbil brain nuclei, 32P incorporation into the NaCl-soluble and HCl-soluble fraction was increased without significant decrease in the phenol-soluble fraction after an ischemic period of 3 h. However, further separation of the phenol-soluble fraction demonstrated decrease of 32P incorporation in protein with high molecular weight and increase in protein with low molecular weight. At present, the significance of these findings, particularly in relation to chromatin template activity or irreversibility of these pathophysiological conditions, is not clear.

A novel SH-type carboxypeptidase in the inner membrane of rat-liver mitochondria

A carboxypeptidase from rat liver mitochondria was partially purified by discontinuous sucrose gradient centrifugation, washes with NaCl/KBr/Tris buffer, and solubilization with 2 M NaCl in the presence of soybean trypsin inhibitor bound to CM-cellulose. By means of dodecylsulfate/polyacrylamide gel electrophoresis a molecular weight of 34,500 was determined; a value of 38,000 was estimated by Sephadex G-100 gel filtration. The carboxypeptidase was completely inhibited by 3 mM Hg2+. In the presence of 3 mM Cu2+ 50% of the catalytic activity was inhibited. Among several peptides tested Cbz-Ala-Phe, Cbz-Leu-Phe, Cbz-Phe-Leu, and Cbz-Phe-Phe, were good substrates. The enzyme activity exhibited a pH optimum of around 9 with Cbz-Ala-Phe as a substrate. After submitochondrial fractionation it was found that the carboxypeptidase is located in the inner mitochondrial membrane.

The localization of an intracellular membrane-bound proteinase from rat liver

To localize the membrane-bound, histone-degrading proteinase, which was previously isolated from the mitochondrial fraction, nuclei, mitochondria, lysosomes, peroxisomes, smooth and rough endoplasmic reticulum, ribosomes and plasma membranes were prepared from a rat liver homogenate according to established methods and characterized by marker enzyme activities. The isolated subcellular fractions were treated with digitonin, and subjected to a discontinuous sucrose gradient centrifugation. The material, which sedimented through 1.74 M surcrose was analyzed in respect to the various marker enzymes and for proteolytic activity. Proteinase activity was found in the material obtained after digitonin treatment and step gradient centrifugation of mitochondria. This finding shows the occurrence of a proteinase in mitochondria; After fractionation of mitochondria into outer and inner membrane, intermembrane fraction and matrix, the proteinase could be localized exclusively in the inner mitochondrial membrane. A possible physiological function of the enzyme during the biosynthesis of inner membrane constituents is discussed.

Protease activities during preparation and handling of nuclear particles containing hnRNA

Conditions were devised to avoid protease activity during the preparation and the subsequent handling of nuclear particles containing hnRNA. During all the steps of preparation of rat liver particles, the presence of phenylmethyl sulfonyl fluoride (PMSF) was required for the reproducibility of the results. It probably inhibited the cellular serine proteases before the separation of the particles from the other cellular structures. Protease activity was detected in the rat liver particles. The enzyme(s) preferentially hydrolyzed a few particle polypeptides. It was not inhibited by PMSF, nor by two trypsin and chymotrypsin-like protease inhibitors, nor by iodoacetamide, but was inhibited by sodium bisulfite and para-hydroxymercury benzoate (PHMB). PHMB was preferred above bisulfite because it could be used at lower concentration. It proved useful when particles were to be incubated at 37 degrees C. A protease hydrolysing the same polypeptides as the liver enzyme was also detected in rat brain particles. However, its activity was much lower in this tissue and the presence of protease inhibitors was not absolutely required under the standard conditions of preparation and handling of brain particles.

Fractionation of purified nucleosomes on the basis of aggregation properties

Monomeric nucleosomes from micrococcal nuclease digest of rat liver nuclei have been purified on Sepharose columns in the presence of divalent cations and 0.6 M NaCl. The particles contain histones H2A, H2B, H3, H4, and a complement of nonhistone chromosomal proteins. In 0.6 M NaCl, the nucleosome mixture sedimented at 10 S; however, when the NaCl was removed approximately 30% of the particles aggregated and precipitated, and the remaining soluble fraction sedimented at 11 S. The aggregation phenomenon was divalent cation-dependent and reversible. Characterization of the macromolecular components of the subfractions of nucleosomes showed that the subfractions differed in composition of species of histone H3 as well as of several nonhistone chromosomal proteins but not in the size of the DNA fragment present. The aggregation properties of the isolated nucleosomes showed similarities to the divalent cation-dependent differences in the extent of chromatin condensation in the intact eukaryote nucleus.

On the competition between protamines and histones: studies directed towards the understanding of spermiogenesis

Specific contacts between the crossinteracting histones effectively prevent their protamine-mediated displacement from nucleoproteins revealing a complex competition pattern not expected from their relative affinities to DNA. Histone H1 is the only species freed under a variety of conditions if native chromatin is used; the results found with chromatins obtained by conventional methods reflect the extent to which histone redistribution has occurred. It is concluded that the events occurring during spermiogenesis must be apt to disrupt histone interactions found in the nucleosomes. The possible role of a protamine phosphorylation/dephosphorylation mechanism is discussed on the basis of comparative experiments with protamines phosphorylated to different extents.

Nuclear proteins. II. Similarity of nonhistone proteins in nuclear sap and chromatin, and essential absence of contractile proteins from mouse liver nuclei

High resolution SDS slab gel electrophoresis has been used to examine the distribution of nonhistone proteins (NHP) in the saline-EDTA, Tris, and 0.35 M NaCl washes of isolated mouse liver nuclei. These studies led to the following conclusions: (a) all the prominent NHP which remain bound to DNA are also present in somewhat similar proportions in the saline-EDTA, Tris, and 0.35 M NaCl washes of nuclei; (b) a protein comigrating with actin is prominent in the first saline-EDTA wash of nuclei, but present as only a minor band in the subsequent washes and on washed chromatin; (c) the presence of nuclear matrix proteins in all the nuclear washes and cytosol indicates that these proteins are distributed throughout the cell; (d) a histone-binding protein (J2) analogous to the HMG1 protein of K. V. Shooter, G.H. Goodwin, and E.W. Johns (Eur J. Biochem. 47:236-270) is a prominent nucleoplasmic protein; (e) quantitation of the major NHP indicates that they are present in a range of 2.2 X 10(5)-5.2 X 10(6) copies per diploid nucleus. Most of the electrophoretically visible NHP are probably structural rather than regulatory proteins; (f) actin, myosin, tubulin, and tropomyosin, if present at all, constitute a very minor fraction of the nuclear NHP. Contractile proteins constitute a major portion of the NHP only when the chromatin is prepared from crude cell lysates instead of from purified nuclei. These studies support the conclusion that there are no clear differences between many nucleoplasmic and chromatin-bound nonhistone proteins. Except for the histones, many of the intranuclear proteins appear to be in equilibrium between DNA, HnRNA, and the nucleoplasm.