Studies on the metabolism of guanidine compounds in mammals

Guanidino compounds in serum, urine, liver, kidney, and brain of man and some ureotelic animals

Guanidino compound levels were quantitatively determined in serum, urine, liver, kidney, and brain of man and of some ureotelic animals. The guanidino compounds were separated over a cation exchange resin, using sodium citrate buffers, and detected with the fluorescence ninhydrin method. Species-specific differences in the levels of some guanidino compounds in the studied ureotelic animals are shown. alpha-Keto-delta-guanidinovaleric acid is a naturally occurring guanidino compound in ureotelic animals, and is not restricted to the pathobiochemistry of hyperargininemic patients. The fasting serum levels observed in beagles are the same as those found in hyperargininemic patients. In serum, liver, and kidney, the homoarginine, beta-guanidinopropionic acid, and gamma-guanidinobutyric acid levels are the highest in rats. The last two compounds have the highest levels of the studied guanidino compounds, with the exception of creatinine, in kidney. Specific high levels of gamma-guanidinobutyric acid and argininic acid are found in brain of rabbits.

Bioactivation mechanism of cytotoxic homocysteine S-conjugates

S-(1,2-Dichlorovinyl)-L-homocysteine is a much more potent nephrotoxin than the corresponding cysteine S-conjugate S-(1,2-dichlorovinyl)-L-cysteine (A. A. Elfarra, L. H. Lash, and M. W. Anders (1986) Proc. Natl. Acad. Sci. USA 83, 2667-2671). The objective of the present experiments was to test the hypothesis that the increased toxicity of homocysteine S-conjugates may be associated with the formation of the reactive metabolite 2-oxo-3-butenoic acid, which may arise via a nonenzymatic retro-Michael elimination reaction from the 2-oxo acid metabolites of homocysteine S-conjugates. S-(2-Benzothiazolyl)-L-homocysteine, which was a substrate for purified bovine kidney cysteine conjugate beta-lyase (glutamine transaminase K) and whose metabolism was dependent on the presence of a 2-oxo acid, was cytotoxic in isolated rat kidney cells and was toxic to rat renal mitochondria, whereas the cysteine S-conjugate S-(2-benzothiazolyl)-L-cysteine had little effect. L-Methionine sulfoximine, L-canavanine, and the Michael acceptor methyl vinyl ketone were cytotoxic. The 2-hydroxy acid analogs of S-(1,2-dichlorovinyl)-L-homocysteine and 2-oxo-3-butenoic acid, S-(1,2-dichlorovinyl)-2-hydroxy-4-mercaptobutanoic acid and 2-hydroxy-3-butenoic acid, respectively, which are expected to be metabolized by rat renal L-2-hydroxy (L-amino) acid oxidase to yield 2-oxo-3-butenoic acid, were also cytotoxic. To obtain evidence for the formation of 2-oxo-3-butenoic acid as a product of the metabolism of L-homocysteine S-conjugates and analogs, trapping experiments were conducted. S-(2-Benzothiazolyl)-L-homocysteine, S-(1,2-dichlorovinyl)-L-homocysteine, L-methionine sulfoximine, and L-canavanine were converted by snake venom L-amino acid oxidase to 2-oxo-3-butenoic acid, which was trapped by the nucleophile methanethiol to yield 4-methylthio-2-oxobutanoic acid; the trapped product was derivatized with 2,4-dinitrophenylhydrazine and was identified by its electronic absorption spectrum and by high-performance liquid chromatography. Similar trapping experiments conducted with kidney homogenates and purified beta-lyase were not successful. The data indicate that the bioactivation of homocysteine S-conjugates and analogs involves the enzymatic formation of the corresponding 2-oxo acids followed by a nonenzymatic retro-Michael elimination reaction to yield the Michael acceptor 2-oxo-3-butenoic acid, which may contribute to the observed cytotoxicity of homocysteine S-conjugates.

Conversion of canavanine to alpha-keto-gamma-guanidinooxybutyrate and to vinylglyoxylate and 2-hydroxyguanidine

It was observed previously that hydroxyguanidine is formed in the reaction of canavanine(2-amino-4-guanidinooxybutanoate) with amino acid oxidases. The present work shows that hydroxyguanidine is formed by a nonenzymatic beta,gamma-elimination reaction following enzymatic oxidation at the alpha-C and that the abstraction of the beta-H is general-base catalyzed. The elimination reaction requires the presence in the alpha-position of an anion-stabilizing group--the protonated imino group (iminium ion group) or the carbonyl group. The iminium ion group is more activating than the carbonyl group. Elimination is further facilitated by protonation of the guanidinooxy group. The other product formed in the elimination reaction was identified as vinylglyoxylate (2-oxo-3-butenoate), a very highly electrophilic substance. The product resulting from hydrolysis following oxidation was identified as alpha-keto-gamma-guanidinooxybutyrate (ketocanavanine). The ratio of hydroxyguanidine to ketocanavanine depended upon the concentration and degree of basicity of the basic catalyst and on pH. In the presence of semicarbazide, the elimination reaction was prevented because the imino group in the semicarbazone derivative of ketocanavanine is not significantly protonated. Incubation of canavanine with 5'-deoxypyridoxal also yielded hydroxyguanidine. Since the elimination reactions take place under mild conditions, they may occur in vivo following oxidation at the alpha-C of L-canavanine (ingested or formed endogenously) or of other amino acids with a good leaving group in the gamma-position (e.g., S-adenosylmethionine, methionine sulfoximine, homocyst(e)ine, or cysteine-homocysteine mixed disulfide) by an L-amino acid oxidase, a transaminase, or a dehydrogenase. Therefore, vinylglyoxylate may be a normal metabolite in mammals which at elevated concentrations may contribute to the in vivo toxicity of canavanine and of some of the other above-mentioned amino acids.

Metabolism of L-[guanidinooxy-14C]canavanine in the rat

The metabolism of L-canavanine, a nonprotein amino acid with significant antitumor effects, was investigated. L-Canavanine, provided at 2.0 g/kg, was supplemented with 5 microCi of L-[guanidinooxy-14C]canavanine (58 microCi/mumol) and administered iv, sc, or orally to female Sprague-Dawley rats weighing approximately 200 g. 14C recovery in the urine at 24 hr was 83, 68, or 61%, respectively, of the administered dose. Another 5-8% of the 14C was expired as 14CO2. The gastrointestinal tract contained 21% of orally administered 14C. Serum, feces, tissues, and de novo synthesized proteins only accounted for a few percent of the original dose by any administrative route. Analysis of the 14C-containing urinary metabolites revealed that [14C] urea accounted for 88% of the urinary radioactivity for an iv injection, 75% for sc administration, and 50% following an oral dose. By all routes of administration, [14C]guanidine represented 5% of the radioactivity in the urine and [14C]guanidinoacetic acid accounted for 2%. Serum and urine amino acid analysis showed a markedly elevated ornithine level. Basic amino acids such as histidine, lysine, and arginine were also higher in the urine. Plasma ammonia levels were determined following oral canavanine doses of 1.0, 2.0, and 4.0 g/kg. A rapid but transient elevation in plasma ammonia was observed only at the 4.0 g/kg dose. This indicates that elevated plasma ammonia is not a likely cause of canavanine toxicity at the drug concentrations used in this study.

Systemic lupus erythematosus-like syndrome in monkeys fed alfalfa sprouts: role of a nonprotein amino acid

Hematologic and serologic abnormalities similar to those observed in human systemic lupus erythematosus (SLE) developed in cynomolgus macaques fed alfalfa sprouts. L-Canavanine sulfate, a constituent of alfalfa sprouts, was incorporated into the diet and reactivated the syndrome in monkeys in which an SLE-like syndrome had previously been induced by the ingestion of alfalfa seeds or sprouts.