The amine specificity of guinea pig liver transglutaminase, a model enzyme for endo-gamma-glutamine:epsilon-lysin transferases, was explored with the aid of synthetic substrates of high apparent affinities. As exemplified by dansyl- (5-dimethylamino-1-naphthalenesulfonyl), (2,4-dinitrobenzenesulfonyl)-, and (2,4,6-triisopropylbenzenesulfonyl)-cadaverines--each of which showed affinities of approximately 4 x 10(7) M-1--the best amine substrates carried a large hydrophobic substituent attached to an alkylamine side chain of about 7.2 A in length. Altogether, our results point to the importance of a hydrophobic binding region in the enzyme from where the alkyl side chain reaches into a narrow crevice toward the active center and positions the primary amine of the substrate for attacking the carbonyl group of the acyl enzyme intermediate.

The acidic dissociation constants of a number of thiols have been determined and collated with those already recorded in the literature.

Peptides patterned on the N-terminal sequence of fibronectin were synthesized and tested for amine acceptor qualities in reactions with dansylcadaverine catalyzed either by coagulation factor XIlla or intracellular transglutaminase (protein-glutamine:amine y-glutamyltransferase, EC 2.3.2.13). On the basis of inverse half-saturations of the enzymes, the order of acceptor substrate affinity for factor XIlla was pEAQQIV >> Boc-AQQIV > Boc-QQIV, and for transglutaminase, Boc-QQIV > Boc-AQQIV > pEAQQIV (amino acid residues are shown in one-letter code; pE, pyroglutamic acid; Boc, tert-butyloxycarbonyl). Sequence analysis of dansylcadaverine-substituted pEAQQIV indicated that the first of the two adjacent glutamine residues was the target of enzymatic modification. Boc-QIV showed no substrate activity with either enzyme. Crosslinking of crystallins in Ca2+-treated rabbit lens homogenate was readily inhibited by Boc-QQIV, Boc-AQQIV, and pEAQQIV, as was the formation of a-chain polymers in human fibrin by pEAQQIV in the presence of human factor XIlla. SDS/PAGE analysis suggested that the inhibitory peptides selectively blocked the electron donor functionalities in these enzymatic crosslinking reactions.Activated fibrin-stabilizing factor (i.e., coagulation factor XIIIa) and the similarly acting transglutaminases (EC 2.3.2.13) bring about the posttranslational crosslinking of protein substrates by promoting the formation of a few Ne-(y-glutamyl)lysine side-chain bridges, with the result of producing a variety of fused homo-and heteropolymeric structures in biological systems (illustrated for dimerization in Scheme 1 on the next page). The kinetic pathway of catalysis by these enzymes, operating with cysteine active centers, is essentially the same as that for proteases such as chymotrypsin, trypsin, or papain, where a Michaelis-type of complexation is followed by the chemical steps of acylation and deacylation. What sets the group of transglutaminases apart from the proteases is a remarkable affinity for the electron-donating second substrate, usually a primary amine, which causes an aminolytic deacylation instead of hydrolysis by water. Unlike with proteases, there is a readily discernible additional complexation of the acylenzyme intermediate with the amine prior to deacylation. This high degree of specificity for the amine made it possible to find synthetic donors [e.g., glycine ethyl ester, N-(5-aminopentyl)-5-dimethylamino-1-naphthalenesulfonamide (dansylcadaverine) (1)] that could compete effectively against the E-amino functionalities of lysine in the natural donor protein (marked in the schemes with an oval) that would otherwise become crosslinked to the acceptor protein (marked with a rectangle). Thus, incorporation of the alternative donor (RNH2; Scheme 2) inhibits crosslinking (Scheme 1) and also leads to the enzymedirected, site-specific labeling of the participating glutaminyl residues in the acceptor protein.Though amine-type of inhibitors of crosslinking were introduced nearly three decades ago and...

N⑀ -(␥-glutamyl)lysine cross-links, connecting various peptide chain segments, are frequently the major products in transglutaminase-catalyzed reactions. We have now investigated the effectiveness of these enzymes for hydrolyzing the ␥:⑀ linkage. Branched compounds were synthesized, in which the backbone on the ␥-side of the cross-bridge was labeled with a fluorophor (5-(dimethylamino)-1-naphthalenesulfonyl or 2-aminobenzoyl) attached through an ⑀-aminocaproyl linker in the N-terminal position, and the other branch of the bridge was constructed with Lys methylamide or diaminopentane blocked by 2,4-dinitrophenyl at the N ␣ position. Hydrolysis of the cross-link could be followed in these internally quenched substrates by an increase in fluorescence. In addition to the thrombin and Ca 2؉-activated human coagulation Factor XIII a , cytosolic transglutaminases from human red cells and from guinea pig liver were tested. All three enzymes were found to display good isopeptidase activities, with K m values of 10 ؊4 to 10Inhibitors of transamidation were effective in blocking the hydrolysis by the enzymes, indicating that expression of isopeptidase activity did not require unusual protein conformations. We suggest that transglutaminases may play a dynamic role in biology not only by promoting the formation but also the breaking of N ⑀ -(␥-glutamyl)lysine isopeptides.Apart from obvious differences in substrate specificities for the ␣-carbonyl groups of endo-Lys and Arg residues by papain (EC 3.4.22.2) and for the ␥-carbonyl groups of certain endo-Gln residues by transglutaminases (EC 2.3.2.13), considerable kinetic and mechanistic similarities exist between these two families of enzymes. Both operate by acylation-deacylation pathways, with a Cys thiol in the catalytic center assisted by a His residue (1-6). However, because of the exceptional specificities of transglutaminases for amines mimicking the ⑀-amino groups of Lys side chains in proteins (7-9), this group of enzymes shows a unique ability for generating protein-to-protein N ⑀ -(␥-glutamyl)lysine cross-links, a post-translational reaction of major biological significance. Transglutaminases are known to participate in various clotting phenomena (7, 10 -16), in the assembly of extracellular matrices (17) and of intracellular polymeric structures in cells under Ca 2ϩ stress (18 -22), and in apoptosis (23).While a great deal of attention has been paid to the amine transferase activities of transglutaminases (3, 4, 24), i.e. to the production of N ⑀ -(␥-glutamyl)lysine bridges and the incorporation of small molecular weight amines into proteins, the isopeptide breaking potential of the enzymes has not yet been explored. Since lack of availability of appropriate substrates may have been a main reason, we embarked on synthesizing ␥-branched peptides with built-in features, which would facilitate the application of fluorescence methodologies for kinetic studies. Two cytosolic transglutaminases of different properties, isolated from human red blood cells (HTg) 1 and fro...

The transglutaminase (protein-glutamine: amine 'y-glutamyltransferase, EC 2.3.2.13)-catalyzed crosslinking of proteins in biological systems can often be inhibited by inclusion of small primary amines or glutamine-containing peptides, which act as site-specific blockers of the relevant acceptor (i.e., glutamine) and donor (i.e., lysine) fumctlonalIties of the natural substrates. Compounds such as dansylcadaverine and dansyl-e-aminocaproyl-Gln-Gln-Ile-Val are particularly useful in sorting out acceptor-donor relationships among lens crystallins. Apart from its fluorescent properties, the dansyl hapten offered special advantages as a "handle" for the rapid isolation of transglutaminase targets even in the complex system oflens cortical homogenate. The dansylated peptide was incorporated into bovine lens proteins under the influence of the Ca2+-activated intrinsic transglutaminase and, after digestion by endoproteinase Glu-C, the tracer-containing fragments were isolated by affinity chromatography on an anti-dansyl antibody column. The major fluorescent peak was isolated by HPLC and sequenced by Edman degradation, which yielded phenylthiohydantoin amino acid derivatives for the first 10 cycles, EKPAVTAAPK, and none for the next 2. The sequence, corresponding to residues 165-174 of aB-crystallin, unambiguously identifies the known carboxyl-terminal domain, EK-PAVTAAPKK, as the prominent lysine-donating fragment in bovine lens.The introduction of short peptides containing a transglutaminase (protein-glutamine:amine y-glutamyltransferase, EC 2.3.2.13)-reactive glutamine residue and an easily recognizable reporter group (e.g., dansyl) made it possible to identify the lysine-carrying (i.e., donor) protein substrates of the enzyme in biological systems (1-4). The peptides fulfilled the dual role of inhibiting protein-protein cross-linking phenomena as they, themselves, became incorporated into the natural donors. Use ofthe compounds in lens homogenates (1-3) revealed a remarkably high degree of selectivity for labeling of proteins under the influence of the Ca2+-activated intrinsic transglutaminase. Moreover, the enzyme-directed, sitespecific labeling reaction could be accomplished with minimum perturbation of the native system under the simple experimental conditions of incubating the tissue homogenate with the tracer peptide in the presence of Ca2+. Following electrophoretic separation, identification ofproteins could be accomplished by detection of fluorescence of the dansyl marker or by blotting with an anti-dansyl antibody. Similar procedures were developed previously for identifying the complementary glutamine-carrying (i.e., acceptor) protein substrates of the enzyme with a fluorescent lysine analogue, dansylcadaverine, as tracer (5).We have now explored in greater depth the potentialities of this general approach of specific tracer recognition and applied it to the isolation of a prominent lysine donor sequence in bovine lens. MATERIALS AND METHODSDansyl-e-aminocaproyl-Gln-Gln-Ile-Val (dns-E-aca-QQIV) was...