Alkylation of protein by methyl methanesulfonate and 1-methyl-1-nitrosourea in vitro

Methylation of horse heart cytochrome c has been examined in vitro with [methyl-14C]methanesulfonate (MMS) and [1-methyl-14C]-1-nitrosourea (MNU) as alkylating agents. Analysis of protein hydrolyzates by an automatic amino acid analyzer indicates that, at pH 9.0 with MMS, epsilon-N-monomethyl-lysine is found to be the only major methylated basic amino acid. On the other hand, the identity of the predominant basic amino acid residue which is [methyl-14C]-labeled by MNU cannot be determined at present. Peptide mapping of chymotryptic digests of cytochrome c after reaction with MMS reveals a lack of specificity in methylation of a specific lysine residue in this hemoprotein.

Enzymatic trimethylation of lysine-72 in cytochrome c

The present observations are the continuation of our earlier study on the physicochemical mechanism of protein-lysine methylation. In this paper the electrophoretic behaviour (pI values) of two chemically modified horse heart cytochromes c at lysine-72 with trifluoromethylphenylcarbamoyl (neutral group) or carboxydinitrophenyl (acidic group) is compared with the enzymatically methylated cytochrome c. The results indicate that although both chemically modified cytochromes c have lower pI values than the unmodified cytochrome c, the enzymatic methylation appears to be much more efficient in lowering the pI values of the protein than the chemical modification. Furthermore, the lowering of the pI value of cytochrome c by enzymatic methylation is highly dependent on the urea concentration. The presence of urea reduces the effect of methylation on the protein molecule and the difference in pI values virtually disappears with the increasing concentration of urea (6 M), which essentially disrupts the protein tertiary structure.

Metabolism of NG-monomethyl-L-arginine

Rat kidney contains an enzyme which hydrolyzes NG-monomethyl-L-arginine to give rise to the formation of ornithine and N-methylurea. Confirmation of formation of these reaction products is carried out colorimetrically as well as radiochemically employing NG-monomethyl-L-[ornithine-14C(U)]arginine. This pattern of reaction products suggests that the enzyme responsible is an arginase type. However, the kidney enzyme is quite distinct from the hepatic arginase; commercial bovine hepatic arginase (L-arginine amidinohydrolase, EC 3.5.3.1) is completely inactive toward NG-monomethyl-L-arginine. Among various rat tissues examined, the hydrolytic activity is the highest in kidney, followed by the activity in liver and pancreas.

Studies on compartmentation of S-adenosyl-L-methionine in Saccharomyces cerevisiae and isolated rat hepatocytes

The existence of metabolically distinct pools of S-adenosyl-L-methionine in Saccharomyces cerevisiae and isolated rat hepatocytes was investigated. Utilizing a relatively long labeling period with [methyl-14C]methionine, a metabolically 'stable' pool was labeled. A subsequent short labeling with [methyl-3H]methionine selectively labeled a putative metabolically 'labile' pool. The existence of these distinguishable pools was ascertained by following the 3H and 14C label disappearance in S-adenosyl-L-methionine during the chase-period in label-free media containing cycloleucine to prevent further synthesis of S-adenosyl-L-methionine. In both yeast and hepatocytes, the 3H/14C ratio in S-adenosyl-L-methionine decreased sharply. The individual 3H and 14C decrease in S-adenosyl-L-methionine showed t1/2 values of 3 and 8 min for yeast and 4 and 18 min for hepatocytes. The results strongly indicate that at least two metabolically distinct S-adenosyl-L-methionine pools actually do exist in both systems. Subcellular fractionation revealed that the 'labile' pool exist in the cytosol for both yeast and hepatocytes while the 'stable' pool exists in the vacuolar and the mitochondrial fraction for the yeast and hepatocytes respectively. The S-adenosyl-L-methionine pools were also studied in normal yeast under anaerobic chase condition and petite mutant yeast. Sharply contrasting with aerobically chased normal yeast, both showed closely parallel 3H and 14C decreases in S-adenosyl-L-methionine.

Cytochrome c specific methylase from wheat germ

The cytochromes c of plants (e.g., wheat germ) possess two trimethyllysines, residues 72 and 86. In order to investigate the nature of these methylations, we have purified a cytochrome c specific methylase S-adenosylmethionine: protein(lysine) N-methyltransferase (protein methylase III) from wheat germ 135-fold. The in vitro site of methylation by both the purified enzyme and crude wheat germ extract toward various forms of horse heart cytochrome c was localized by two dimensional peptide mapping, Aminex A-5 column peptide analysis, and CNBr cleavage analysis to be the residue 72 lysine. However, no additional sites, in particular residue 86, were seen to be methylated. Although the enzyme is highly specific toward cytochrome c as an in vitro protein substrate, avian cytochromes c are seen to be much better substrates than those from mammalian sources. The enzyme possesses an extremely low Km for apocytochrome c (1.21 microM), suggesting that methylation may occur before heme attachment in vivo. Various S-adenosyl-L-homocysteine analogues were tested for their inhibitor capability toward the enzyme; it was observed that only the D and L forms of S-adenosylhomocysteine are inhibitors while analogues modified in the adenine or homocysteine moieties do not possess inhibitory capability. Results from the Aminex A-5 column chromatography of horse heart cytochrome c chymotryptic digest showed the N epsilon-methyl-, N epsilon-dimethyl-m and N epsilon-trimethyllysine forms of the residue 68-74 peptide to elute earlier than the unmethylated form. This results suggest that the methylated peptides are less basic than the unmethylated form.

Reduced erythrocyte membrane protein methylation in sickle cell anemia

The methylation of erythrocyte membrane components in sickle cell anemia has been studied and found to differ considerably from that of normal erythrocytes. When sickle erythrocytes were incubated under physiological conditions (pH = 7.4, 37 degrees C) in the presence of L-[methyl-3H]methionine or S-adenosyl-L-[methyl-3H] methionine, a 50% decrease in the protein-carboxyl methylation was observed compared to the normal erythrocyte. This reduction in degree of methylation was reflected in all of the major methylated protein bands on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Since this methylation is catalyzed by the cytosolic protein methylase II (S-adenosylmethionine:protein-carboxyl O-methyltransferase, EC 2.1.1.24), the in vitro substrate capability of sickle erythrocyte ghosts and pH 11 treated ghosts were tested by incubation with purified protein methylase II and S-adenosyl-L-[methyl-14C]methionine. Both types of sickle cell ghost preparations showed the same 50% decrease in methylation as was seen in the intact cells. Since it was also shown that the protein methylase II and methyl acceptor membrane protein levels in the sickle erythrocytes are the same as the normal control, the data suggest that the observed reduced methylation may be due to an altered membrane conformation. The methylation of phospholipid was also studied and found to be decreased in sickle cell erythrocytes. However, this methylation was extremely minor in comparison to protein-carboxyl methylation which represents the bulk of the membrane methylation.

Effect of methylation on the stability of cytochrome c of Saccharomyces cerevisiae in vivo

The in vivo stability of methylated and unmethylated cytochrome c in Saccharomyces cerevisiae was studied by pulse-labeling the hemoproteins with [methyl-3H]-methionine and/or [2-14C]methionine and following the fate of these proteins under anaerobiosis and in the presence of cycloleucine. These two conditions will respectively block further cytochrome c synthesis and inhibit methylation by lowering the cellular S-adenosyl-L-methionine pool and, thus, permit an unambiguous interpretation of the data. The results showed that the rate of degradation of unmethylated cytochrome c was constant throughout the chase period, while methylated cytochrome c degradation was seen only in the later part of cold chase. At the end of the chase period (40 h), the extent of degradation of the unmethylated species was three times higher than the methylated species. This indicated that the methylation of cytochrome c has a protective effects against the intracellular proteolytic enzyme attack on itself. Furthermore, this protective effect was considerably reduced in the petite mutant, which lacks high affinity cytochrome c binding sites, functional cytochrome c reductase, and oxidase, and possesses a less integrated and organized mitochondrial membrane. These results led us to the conclusion that the mechanism of methylated cytochrome c stabilization is best explained by a higher efficacy of binding to the mitochondria.

Metabolism of NG-monomethyl-L-arginine: formation of NG-methylagmatine by Escherichia coli preparation

Commercially obtained Escherichia coli preparation degrades NG-monomethyl-L-arginine derived from methylation of protein-bound arginine residues. Elemental analysis and NMR spectroscopy established the reaction product as NG-methylagmatine, a decarboxylation product of NG-monomethyl-L-arginine.

Microsome-dependent methylation of erythrocyte proteins by dimethylnitrosamine

The precarcinogen, N-[methyl-14C]dimethylnitrosamine, rat liver microsomes and NADPH were incubated in the presence of intact human erythrocytes. Uptake of radioactivity into erythrocytes was time-dependent and analysis of an acid hydrolysate of the erythrocyte proteins showed several radioactive peaks which were identified as 3-N-methyl- and 1-N-methyl-histidine, S-methyl-cysteine. Since no binding occurs in the absence of microsomes, the alkylating species from dimethylnitrosamine metabolism can clearly penetrate the erythrocyte plasma membrane.

In vivo studies on yeast cytochrome c methylation in relation to protein synthesis

Methylation of cytochrome c was studied in vivo using double label with L-[methyl-3H]methionine and DL-[2-14C]methionine. In pulse-chase experiments the cytochrome c associated with the mitochondrial fraction possessed a higher ratio of 3H/14C label, suggesting the presence of methylated cytochrome c. The appearance of methylated cytochrome c in mitochondria showed no lag phase. The inhibition of cytochrome c methylation in presence of cycloheximide indicated that both the methylation and protein synthesis were tightly coupled and cycloheximide selectively inhibited cytochrome c methylation. There was also an indication of selective turnover of incorporation methyl groups in preformed cytochrome c.

Enzymatic trimethylation of residue-72 lysine in cytochrome c. Effect on the total structure

A highly purified protein methylase III from Neurospora crassa or Saccharomyces cerevisiae specifically methylates a single lysine residue of position 72 of horse heart cytochrome c. The enzymatically methylated cytochrome c has been separated from the unmethylated counterpart species by isoelectric focusing. Simultaneously, the pI values of these two species were found to be 9.49 and 10.03, respectively. Since methyl substitution increases the basicity associated with the epsilon-amino group of lysine residues, the observed decrease in pI value is in opposition to the predicted increase. Space-filling models revealed the possibility of a hydrogen bond between the oxygen of amide of residue-70 asparagine and the epsilon-amino nitrogen of residue-72 lysine in unmethylated horse heart cytochrome C. the enzymatic methylation of residue-72 lysine tends to dissociate this hydrogen bond, thereby possibly inducing the shift of 'effective charge' of the protein molecule. This paper also deals with the pI values of cytochromes c from 13 different sources, determined by the isoelectric focusing technique.

In vivo carboxyl methylation of human eruthrocyte membrane proteins

In order to study the in vivo methylation of ghost membrane proteins, human erythrocytes were incubated with L-[methyl-3H]methionine. The [3H]methyl incorporation into the membrane components was observed in freshly prepared erythrocytes. Upon treatment of the methylated membrane at pH 7.4, 100 degrees C for 5 min, a condition which is known to hydrolyze protein-carboxyl methyl ester, 80% of the incorporated [3H]methyl was recovered as methanol. Cycloleucine, an inhibitor for S-adenosylmethionine synthetase, inhibited 70% of the methylation. The analyses of the methylated proteins by sodium dodecyl sulfate/polyacrylamide gel electrophoresis revealed two major methylated protein peaks which were tentatively identified as glycophorin A and band 4.5. This methylation pattern is similar to the in vitro methylation pattern when purified human ghosts were methylated with purified protein methylase II and S-adenosylmethionine (Galletti, P., Paik, W. K., and Kim, S. (1979) Eur. J. Biochem. 97, 221-227). It is concluded that carboxyl methylation of erythrocyte membrane proteins is the major methylation reaction in vivo in these membranes.

Selective methyl esterification of erythrocyte membrane proteins by protein methylase II

Methyl esterification of erythrocyte membrane proteins have been demonstrated by incubating the isolated membrane with purified protein methylase II (S-adenosyl-methionine:protein-carboxyl O-methyltransferase, EC 2.1.1.24) and S-adenosyl-L-[methyl-14C]methionine. Methyl esterification of membrane-bound proteins occurred selectively to proteins corresponding to bands 3 (mol wt 97 000), 4 (mol wt 75 000), and 4.5 (mol wt 48 000) [designated according to Steck, T. L. (1974), J. Cell Biol. 62, 1] as identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Mild alkali treated depleted vesicles which lacked bands 1, 2, 5, and 6 had a higher methyl accepting capacity; 500 pmol of methyl groups/mg of depleted vesicle proteins vs. 200 pmol of methyl groups/mg of intact membrane proteins. Alkali-extractable membrane components were not methylated.