Separation and identification of dipeptides by paper and column chromatography

Purification and characterization of a small dermatan sulphate proteoglycan implicated in the dilatation of the rat uterine cervix

A small dermatan sulphate proteoglycan (DSPG) was extracted from rat cervices and was purified by using DEAE-Sephacel ion-exchange chromatography, gel filtration on Sepharose CL-2B and CsCl-density-gradient centrifugation. Sedimentation-equilibrium centrifugation gave a weight-average Mr of 95,000. Amino acid analysis showed a high content of aspartic acid, glutamic acid, glycine and leucine. The glycosaminoglycan chains had Mr 50,000 as determined by gel filtration. Chondroitin AC lyase and chondroitin ABC lyase digestions of these chains showed that they were composed of 75% dermatan sulphate and 25% chondroitin sulphate. Chondroitin ABC lyase digestion produced a core protein of Mr 45,000. The core protein, after treatment with HF, had Mr 37,000. Amino acid sequences of the N-terminus and a CNBr-cleavage peptide showed similarity to the sequences of core proteins of small proteoglycans of bovine and human origin, but the N-terminal glycosaminoglycan-attachment site (Ser-Gly-Ile-Ile) differed from the consensus sequence (Ser-Gly-Xaa-Gly) [Bourdon, Krusius, Campbell, Schwartz & Ruoslahti (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 3194-3198]. A polyclonal antibody against the rat cervical DSPG reacted with small proteoglycans from cervices of human, mouse, dog, cow and sheep. DSPG is the major proteoglycan species present in the cervix. The amount of DSPG per cervix increases 4-fold during pregnancy, then falls precipitously within 1 day post partum. A role in cervical dilatation is postulated for this proteoglycan.

Catalysis of peptide bond formation by histidyl-histidine in a fluctuating clay environment

The condensation of glycine to form oligoglycines during wet-dry fluctuations on clay surfaces was enhanced up to threefold or greater by small amounts of histidyl-histidine. In addition, higher relative yields of the longer oligomers were produced. Other specific dipeptides tested gave no enhancement, and imidazole, histidine, and N-acetylhistidine gave only slight enhancements. Histidyl-histidine apparently acts as a true catalyst (in the sense of repeatedly catalyzing the reaction), since up to 52 nmol of additional glycine were incorporated into oligoglycine for each nmol of catalyst added. This is the first known instance of a peptide or similar molecule demonstrating a catalytic turnover number greater than unity in a prebiotic oligomer synthesis reaction, and suggests that histidyl-histidine is a model for a primitive prebiotic protoenzyme. Catalysis of peptide bond synthesis by a molecule which is itself a peptide implies that related systems may be capable of exhibiting autocatalytic growth.

Separation and sequence of dipeptides using gas chromatography and mass spectrometry of their trimethylsilylated derivatives

Dipeptides containing all 20 common amino acids in the amino and carboxyl positions were trimethylsilylated under a variety of conditions. The dipeptides (about 200 of the possible 400) were synthesized by conventional means or derived from polypeptides digested with dipeptidyl aminopeptidase. Derivatives were prepared in high yield by reaction with N,O-bis(trimethylsilyl)trifluoroacetamide in acetonitrile. Reaction products were identified by mass spectrometry. Numerous stationary phases and supports were compared in the gas chromatographic analysis of the derivatives. Best results were obtained with a 0.2 X 60 cm column packed with Chromosorb 750 coated with a 1% solution of OV-1. All Me3Si dipeptides except those containing arginine eluted in less than 30 min when the column was temperature programmed from 100 to 280 degrees C. The typical mass spectrum of trimethylsilylated dipeptides contains two significant ions used for identification: the ion derived from the amino terminal residue by beta cleavage of the central CH-CO bond and the molecular ion minus a methyl group. Other useful ions also have been noted. A sufficient variety of samples have been examined to establish predictable fragmentations which should allow the identification of all 400 possible dipeptides.

Acyl and amino intermediates in reactions catalysed by pig pepsin. Analysis of transpeptidation products

The action of pig pepsin on a variety of small peptides including Leu-Trp-Met-Arg, Leu-Trp-Met, Leu-Leu-NH2, benzyloxycarbonyl-Phe-Leu and Gly-Leu-Tyr was studied. Leu-Leu-Leu was found to be the major product from the substrates Leu-Trp-Met-Arg and Leu-Trp-Met, indicating that the predominant reaction at pH 3.4 was a transpeptidation of the acyl-transfer type. Leu-Leu-Leu was also formed in high yield by amino transfer from benzyloxycarbonyl-Phe-Leu. Like the amino-transfer reactions the acyl transfer proceeded via a covalent intermediate, since [14C]leucine was not incorporated into transpeptidation products and did not exchange with enzyme-bound leucine in the presence of acceptors. With Leu-Trp-Met both acyl and amino transpeptidation products, namely Leu-Leu, Leu-Leu-Leu, Met-Met and Met-Met-Met, were formed in addition to methionine and leucine. With Leu-Trp-Met-Arg (1 mM) the pH optimum for the rates of hydrolysis and acyl transfer is about pH 3.4. At this pH the rate of acyl transfer exceeds that of hydrolysis; at pH 2, however, hydrolysis was faster than transfer. A comparison of the effect of the length of substrates and products on the reaction rates allows the conclusion that the binding site can extend over eight to nine amino acid residues. Although the experiments provide no conclusive evidence for or against the involvement of amino and/or acyl intermediates in the hydrolysis of long peptides and proteins, the high yield of transpeptidation reactions of both types observed with some substrates suggests a major role for the intermediates in pepsin-catalysed reactions. The results also show that when pig pepsin is used for the digestion of proteins for sequence work, the likelihood of the formation of transpeptidation products is considerable. In this way peptides not present in the original sequence could easily form in a reasonably good yield.

Acyl intermediates in penicillopepsin-catalysed reactions and a discussion of the mechanism of action of pepsins

Penicillopepsin catalyses transpeptidation reactions involving the transfer of the N-terminal amino acids of suitable substrates via covalent acyl intermediates to acceptor peptides, usually the substrate. The major products obtained when Phe-Tyr-Thr-Pro-Lys-Ala and Met-Leu-Gly were used as substrates were Phe-Phe and Met-Met respectively. With Met-Leu-Gly the tetrapeptide Met-Met-Leu-Gly was observed as probable intermediate. Co-incubation of Leu-Tyr-Leu and Phe-Tyr-Thr-Pro-Lys-Ala led to the formation of Leu-Phe and Phe-Leu as well as Leu-Leu and Phe-Phe. No reaction was observed with tripeptides in which the first or second amino acid is glycine. It appears that two amino aicds with large hydrophobic residues are needed for the transpeptidation reaction. Nucleophilic compounds other than peptides, such as hydroxylamine, aliphatic alcohols and dinitrophenylhydrazine, were not acceptors for the acyl group. Leucine, phenylalanine and leucine methyl ester also had no effect on the reaction. The transpeptidation reaction proceeded readily at pH 3.6 and 4.7. At pH 6.0 the reaction was slow and at pH 1.9 little or no transpeptidation was observed. Porcine pepsin catalyses similar transpeptidation reactions. Sequence studies show that porcine pepsin and penicillopepsin are homologous. The present study also suggests that they have a very similar mechanism. Evidence available at this time indicates that the mechanism of these enzymes is complex and may be modulated by secondary substrate-enzyme interactions. A hypothesis is presented which proposes that pepsin-catalysed reactions proceed via different covalent intermediates (amino-intermediates or acylintermediates) depending on the nature of the substrate. The possibility that some reactions do not involve covalent intermediates is also discussed.

An improved technique for the analysis of amino acids and related compounds on thin layers of cellulose. X. The characterization of some methionyl, phenylalanyl, tyrosyl and other peptides by thin-layer and ion-exchange chromatography

This paper is a continuation of previous work (Parts VI, VIII, and IX) designed to identify small peptides in biological fluids by a combination of ion-exchange and thin-layer chromatography. Several series of peptides, mainly dipeptides, with methionine, phenylalanine, tyrosine, aspartic acid, serine, or glutamic acid as the N-terminal amino acid, have been examined.