The biosynthesis of 2-guanidinoethanol in intact mice and isolated perfused rabbit kidneys

Synthesis of neuroactive guanidino compounds by rat kidney L-arginine: glycine amidinotransferase

Several neuroactive guanidino compounds have been reported to be synthesized in mammals by transamidination reactions. The enzyme(s) responsible for their synthesis and their location in the body has not been well established. The purpose of this investigation was to determine if purified homogeneous rat kidney alpha- and beta-L-arginine : glycine amidinotransferase (transamidinase) would catalyze the synthesis of certain neuroactive guanidino compounds, and if so, to determine if any catalytic specificity existed between the two forms of the enzymes. L-Arginine (Arg) was used as the amidino group donor and the following compounds were investigated for their ability to accept the amidino group: ethanolamine; 4-aminobutyric acid; lysine; 5-aminovaleric acid; 3-aminopropionic acid; taurine; L-glutamic acid (Glu); L-aspartic acid (Asp); and histidine (His). All of the above listed compounds served as amidino group acceptors for the enzyme except Glu, Asp and His. No differences were found between the alpha- and beta-transamidinase in any of the experiments reported, and the synthesis of 2-guanidinoethanol by the enzyme was by a sequential mechanism with a Km for Arg and ethanolamine of 14mM and 163mM, respectively. The possibility that the site of synthesis of the neuroactive guanidino compounds in the kidney and perhaps pancreas is discussed.

2-Guanidinoethanol increased dopamine release and 3,4-dihydroxyphenylacetic acid content, but not homovanillic acid content in the rat brain: electroneurochemical and enzymological studies

The effects of 2-guanidinoethanol (GEt) on the release of monoamines and on the activity of their degrading enzymes were studied in order to investigate why 3,4-dihydroxyphenylacetic acid (DO-PAC) increased to a much greater extent than homovanillic acid (HVA) after GEt injection into rat brain. In differential pulse voltammograms recorded using an electrochemically treated carbon fiber electrode, two distinct oxidation peaks, one at 130mV (DOPAC peak) and the other at 300 mV (5-hydroxyindoleacetic acid (5-HIAA) peak), were observed. In the hippocampus, the DOPAC peak increased markedly compared to the peak height recorded prior to the intracerebroventricular injection of GEt (6 mumol). Although the DOPAC peak height increased to 350% 4 hours after GEt injection, the 5-HIAA peak showed no change. In the striatum, the DOPAC peak increased to 150% 3 hours after GEt injection. Serial changes in the extracellular levels of DOPAC, HVA, and 5-HIAA were monitored in the striatum after GEt injection, using an in vivo brain micro-dialysis technique. Although the DOPAC levels strated to increase 80 minutes after GEt injection, HVA and 5-HIAA levels showed no change. On the other hand, monoamineoxidase, which metabolizes dopamine to DOPAC, was not activated and catechol-O-methyltransferase, which metabolizes DOPAC to HVA, were not inhibited by 5 mM of GEt in vitro. These data suggested that GEt increased the release of dopamine, but not of serotonin, and that GEt might restrict the DOPAC transport system.

Effect of 2-guanidinoethanol on levels of monoamines and their metabolites in the rat brain

The contents of monoamines and their metabolites in rat brains 3 hours after the intracerebroventricular injection of 6 mumol of 2-guanidino-ethanol (GEt) were measured by HPLC. GEt which is a configurational analogue of 4-aminobutanoic acid (GABA) induced severe running fits and tonic-clonic convulsions as well as epileptic discharges. In GEt-administered rats, dopamine (DA) decreased in the cortex, hippocampus and hypothalamus. 3,4-Dihydroxyphenylacetic acid (DOPAC) increased to about the same level in all brain regions, therefore the distribution of DOPAC appeared to be homogeneous in the brain. The homovanillic acid levels also increased in the striatum and hippocampus. No significant change in the norepinephrine contents was observed in any region. The turnover ratio of DA increased significantly except in the striatum. Serotonin levels increased in the hypothalamus and midbrain by GEt administration, though 5-hydroxyindoleacetic acid levels showed no change in any of the brain regions. These data suggest that the activity of dopaminergic and serotonergic neurons are increased by GEt.

Formation of 2-guanidinoethanol by a transamidination reaction from arginine and ethanolamine by the rat kidney and pancreas

The formation of 2-guanidinoethanol (GEt) from L-arginine (Arg) and ethanolamine (EA) was studied using rat kidney homogenates. Maximum GEt formation was observed between pH 8.7 and 9.1, and the enzyme catalyzing the GEt synthesis was stable between pH 5.6 and 9.1. The rate of GEt formation from Arg and EA by rat kidney homogenates obeyed simple Michaelis-Menten type kinetics. L-Ornithine and glycine inhibited GEt formation by rat kidneys. Both of them inhibited GEt formation in a linear mixed-type inhibitory manner when Arg concentrations were varied at a fixed concentration of EA, while they showed competitive inhibition when EA concentrations were varied at a fixed concentration of Arg. L-Canavanine and guanidinoacetic acid as well as Arg acted as an amidine donor for GEt formation, but L-homoarginine, 3-guanidinopropionic acid and 4-guanidinobutyric acid did not. GEt synthesis was also observed in the rat pancreas. It had almost half of the activity of rat kidney to form GEt. This ratio of kidney to pancreas was approximately equal to that of L-arginine:glycine amidinotransferase (transamidinase, EC 2.1.4.1) in kidney and pancreas. These results suggest that GEt may be synthesized from Arg and EA by a transamidinase catalyzing reaction.