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Studies on the Biosynthesis of Mitochondrial Malate Dehydrogenase and the Location of Its Synthesis in the Liver Cell of the Rat by R

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Studies on the Biosynthesis of Mitochondrial Malate Dehydrogenase and the Location of Its Synthesis in the Liver Cell of the Rat by R Biochem. J. (1972) 126, 211-215 211 Printed in Great Britain Studies on the Biosynthesis of Mitochondrial Malate Dehydrogenase and the Location of its Synthesis in the Liver Cell of the Rat By R. W. BINGHAM* and P. N. CAMPBELL Department ofBiochemistry, University ofLeeds, 9 Hyde Terrace, Leeds LS2 9LS, U.K. (Received 9 August 1971) A method is described for the isolation of mitochondrial malate dehydrogenase from either the whole tissue homogenate or from the microsomal fraction of rat liver. The procedure involves the treatment of the tissue extract with detergent followed by gel filtration and chromatography on Amberlite CG-50 and DEAE-cellulose. The resulting enzyme was homogeneous by the criterion of gel electrophoresis. Incubation of the microsomal fraction from rat liver under the usual conditions for protein synthesis in the presence of [3H]leucine resulted in the incorporation of 3H into the mitochondrial malate dehydrogenase when purified as described. The results are taken to indicate that the mito- chondrial enzyme is synthesized by the cytoplasmic ribosomes. Possible ways in which the cytoplasmic and mitochondrial forms ofmalate dehydrogenase reach their final locations in the cell are discussed. The balance of evidence at present available indi- which the protein moves from the cytoplasmic ribo- cates that only some of the structural proteins of the somes to the mitochondria. In this respect malate mitochondria are synthesized within the organelle dehydrogenase is of particular interest as it exhibits and that most of the proteins, including those that are dual location in the cell. In various tissues, including soluble, are of extra-mitochondrial origin (see, e.g., rat liver, there are two major forms of the enzyme, Roodyn & Wilkie, 1968; Ashwell & Work, 1970). one exclusive to the cytoplasm and one to the mito- As an example of a soluble mitochondrial protein the chondria. These two forms have very different phy- biosynthesis of cytochrome c has been studied ex- sical and chemical properties, but their substrate tensively. From kinetic studies with Krebs ascites- specificities are very similar (Thorne et al., 1963; tumour cells Freeman et al. (1967) suggested that Thorne, 1968; Mann & Vestling, 1968). Thus if both cytochrome c was synthesized extra-mitochondrially. forms ofthe enzyme are synthesized at the same loca- This conclusion was confirmed by kinetic studies on tion, some mechanism must exist for the exclusive liver, after the injection of "'C-labelled amino acids movement of one form into the mitochondrion while into rats, by Gonzilez-Cadavid & Campbell the other is released into the cytoplasm. (1967a,b) and by Kadenbach (1969, 1970). Further In the present paper we report a method suitable confirmation was obtained by Penniall & Davidian for the purification of rat liver mitochondrial malate (1968) who used [3H]aminolaevulinic acid as a pre- dehydrogenase, and its application to a study of the cursor for haem. synthesis of the enzyme by isolated liver microsomal In their study on the incorporation of "4C-labelled preparations. amino acids into protein by isolated rat liver mito- chondria Roodyn et al. (1962) showed that most of the incorporation was into insoluble protein and there Materials and Methods was little into either cytochrome c or malate dehydro- Materials genase. We decided to study the synthesis ofthe latter enzyme for two reasons. First, we hoped to demon- Chemicals. ATP (disodium salt), GTP (sodium strate the synthesis of malate dehydrogenase by a salt), NAD, 3(4,5-dimethylthiazolyl-2)-2,5-diphenyl microsomal preparation and so locate its site of syn- tetrazolium bromide and tris buffer (Sigma 7-9 grade) thesis more certainly than would be possible by kinetic were from Sigma Chemical Co., St. Louis, Mo., studies. This approach had not been possible with U.S.A. Phosphoenolpyruvate (potassium salt) was cytochrome c, owing perhaps to the covalent attach- from C. F. Boehringer und Soehne G.m.b.H., ment of the haem moiety to the apoprotein, which Mannheim, Germany. 2-Amino-2-methylpropan-1-ol could not be effected by the isolated microsomal and NN,NN'N-tetramethylethylenediamine were fraction. Secondly, it is of interest to study the way in from Koch-Light Laboratories Ltd., Colnbrook, * Present address: Division of Biological Sciences, Bucks., U.K. DEAE-cellulose, grade DE23, was University of Warwick, Coventry, CV4 7AL, U.K. from W. and R. Balston (Modified Cellulose) Ltd., Vol. 126 212 R. W. BINGHAM AND P. N. CAMPBELL Springfield Mill, Maidstone, Kent, U.K. Soluene X-100 supernatant from the microsomal incubations TM. 100 was from Packard Instruments, Middlesex, or it was prepared from whole rat liver homogenized U.K. Starch, hydrolysed for gel electrophoresis in 3vol. of 0.025M-potassium phosphate buffer, (lot 228-1), was from Connaught Medical Research pH 7.0, containing 0.5% (w/v) of Triton X-100. This Lab., Toronto, Canada. homogenate was centrifuged at 105OO0g for 60min Radiochemicals. L-[4,5-3H]Leucine (specific radio- to remove the particulate material not solubilized by activity 19.7Ci/mmol) was purchased from The the Triton X-100. The Triton X-100 supernatant Radiochemical Centre, Amersham, Bucks., U.K. was passed through a column of Sephadex G-25 Animals. The rats used in these studies were male, equilibrated with 0.025M-potassium phosphate Wistar strain, and were purchased from Tuck and buffer, pH7.0 (the size of the column was such that Sons Ltd., Rayleigh, Essex, U.K. the sample was equivalent to about 10% of the column volume). This and all subsequent steps were Isolation of the microsomal fraction and preparation done in a cold-room at 1-5°C. of cell sap The Sephadex-treated sample was loaded on a column (1.5 cm x 12cm) ofAmberlite CG-50 that had Rats were starved overnight, killed by a blow on the been equilibrated with 0.025M-potassium phosphate head and the livers removed. The livers were minced, buffer, pH7.0. The resin had previously been pre- disrupted in medium A [0.15M-sucrose-25mM- treated as described by Hagihara et al. (1958). The KCl-lOmM-MgSO4-34.5mM-tris (adjusted to pH7.8 fraction that passed through the column unbound with HCI at 20°C)] and the microsomal fraction was contained, among other proteins, the cytoplasmic recovered exactly as described by Ragnotti et al. malate dehydrogenase. The column was washed with (1969). Cell sap was also prepared and treated with a large volume (2 litres) of the 0.025M-phosphate Sephadex G-25 as described in the latter paper. buffer and the protein was eluted with 0.2M-potas- sium phosphate buffer, pH7.0. This eluted a deep-red Incubation procedure fraction (25ml) containing the mitochondrial enzyme. The incubation procedure was as follows: In a The eluate was passed through a column (2.5cm 250ml flask the following solutions were assembled: x 25cm) of Sephadex G-25 equilibrated with 0.02M- L-[3H]leucine (5ml = 500,uCi); l0OmM-phosphoenol- tris-HCl buffer, pH8.3 at 20°C. The eluate was di- pyruvate (15ml); lO0mM-ATP (2ml); 5mM-GTP alysed overnight against the same tris buffer. The (5ml); Sephadex G-25-treated cell sap (20ml); micro- fraction was then loaded on a column (1.5cm x 12cm) somal suspension (40ml); medium A (13 ml). For a of DEAE-cellulose (Whatman DE23, pretreated as control incubation, ATP was omitted and the volume recommended in the Whatman Technical Bulletin) ofmedium A increased accordingly. The microsomal equilibrated with 0.02M-tris-HCl buffer, pH8.3, suspension was added and the contents of the flask which was also the eluting buffer. Most of the pro- were thoroughly mixed and divided between five teins remained bound to the DEAE-cellulose in a 5Oml conical flasks. These flasks were incubated, red band but the mitochondrial malate dehydro- with constant shaking, in a water bath at 37±0.30C genase passed through unretained in a colourless for 30min. At the end of the incubation period, the fraction. contents of the flasks were pooled, then Triton X-100 was added to a final concentration of 1 % and the Electrophoresis ofmitochondrialmalate dehydrogenase mixture rapidly frozen for storage overnight at -12°C. The next day the mixture was thawed and The purity of the isolated enzyme was determined centrifuged at 105000g for 60min in an M.S.E. by gel electrophoresis either with polyacrylamide gel Superspeed 65 centrifuge. The supernatant was or starch. The polyacrylamide-gel system was similar used for the isolation and purification of malate to the pH8.9 system of Davis (1964). The spacer and dehydrogenase. sample gels used by Davis (1964) were not employed, 50-100,u of enzyme sample (containing 5% sucrose Purification ofmitochondrial malate dehydrogenase plus a trace of Bromophenol Blue) being layered directly on the top of the running gel. The samples Initially purification of the enzyme was attempted were run at 4mA/tube (350-400V) until the Bromo- by the method of Roodyn et al. (1962). The method phenol Blue band reached the bottom of the tube was not very reproducible in our hands, and the (usually 45-75min) and sometimes the samples were method finally adopted was based on that described run for a longer time, as the malate dehydrogenase by Thorne & Dent (1968). This method gives a com- has a low mobility. Gels were stained for protein plete separation of cytoplasmic and mitochondrial (Chrambach et al., 1967) or for malate dehydrogenase malate dehydrogenase in one step by using a column activity (Davidson & Cortner, 1967). of Amberlite CG-50. Starch-gel electrophoresis was performed by the The crude enzyme sample was either the Triton method of Fine & Costello (1963) by using a gel 1972 BIOSYNTHESIS OF MALATE DEHYDROGENASE 213 15cm x 15cm, 5mm thick.
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