1,2,3,4-Tetrahydro-2-methyl-4,6,7-isoquinolinetriol inhibits tyrosine hydroxylase activity in rat striatal synaptosomes.

1,2,3,4-tetrahydro-2-methyl-4,6,7-isoquinolinetriol (TMIQ), a tetrahydroisoquinoline derivative of adrenaline, was tested for potency as an analog of the dopamine depleting agent 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in assays of tyrosine hydroxylase (TH) activity in the striatal synaptosome preparation. TMIQ inhibited TH activity with an IC50 (4 x 10(-6)M) similar to that found for MPTP (IC50 1 x 10(-6)M). TH inhibitions produced by IC50 concentrations of TMIQ were reversed by monoamine oxidase (MAO)-A or MAO-B inhibitors (clorgyline or deprenyl), or the dopamine reuptake blocker nomifensine, or excess cofactor (6R)-5,6,7,8-tetrahydro-L-biopterin. TMIQ did not appear to act at the presynaptic D2 sulpiride sensitive autoreceptor for dopamine synthesis modulation. These in vitro data are consistent with earlier findings that TMIQ acts as a dopamine depleting agent, and with the possibility that TMIQ may have a degree of MPTP-like activity in vivo.

1,2,3,4-Tetrahydro-2-methyl-4,6,7-isoquinolinetriol depletes catecholamines in rat brain.

1,2,3,4-Tetrahydro-2-methyl-4,6,7-isoquinolinetriol (TMIQ) was synthesised and tested for activity as a dopamine-depleting agent in rat brain. After intracerebroventricular infusion, TMIQ caused reductions in dopamine concentrations in substantia nigra, striatum, hypothalamus, and dorsal raphe, and reduction in noradrenaline concentrations in locus coeruleus. TMIQ also reduced 5-hydroxytryptamine concentrations in dorsal raphe and substantia nigra, although with a lower potency. Comparisons between TMIQ and MPTP showed that they were approximately equipotent in depleting dopamine in the substantia nigra, hypothalamus, and dorsal raphe. Pretreatment of animals with a combination of monoamine oxidase A and B inhibitors completely prevented the TMIQ-induced reductions in dopamine concentrations in substantia nigra and hypothalamus. Direct unilateral intrastriatal injections of TMIQ produced marked ipsilateral reductions in striatal dopamine, correlating with a behavioural response consisting of turning towards the side of injection. The results suggest that TMIQ should be evaluated further as a possible MPTP-like compound, which may derive from endogenous beta-hydroxylated catecholamines.

The contribution of charge to affinity at functional (M3) muscarinic receptors in guinea-pig ileum assessed from the effects of the carbon analogue of 4-DAMP methiodide.

1. 4-Diphenylacetoxy-1:1-dimethyl cyclohexane (carbo-4-DAMP) is the carbon analogue of 4-diphenylacetoxy-N-methylpiperidine (4-DAMP) methiodide. The compounds differ only in that the quaternary nitrogen atom in 4-DAMP methiodide is replaced by a quaternary carbon atom, which is uncharged. 2. Carbo-4-DAMP appears to act competitively at functional (M3) muscarinic receptors in guinea-pig ileum. Estimates of log affinity constant are 6.0 at 30 degrees C and 5.9 at 37 degrees C, i.e. the compound has 0.1% of the affinity of 4-DAMP methobromide. 3. The absence of charge makes little difference to the conformation as determined by X-ray crystallography. The bond lengths and angles are very similar, though the bonds in the cyclohexane ring of carbo-4-DAMP are consistently slightly longer than those in the piperidinium ring of 4-DAMP methiodide, and the presence of the charge slightly reduces the space between molecules. 4. The difference between the affinities of 4-DAMP methobromide and carbo-4-DAMP indicates that the contribution of coulombic forces to the binding between 4-DAMP methiodide and muscarinic (M3) receptors is at least 17 kJ mol-1 (4.1 kcal mol-1) at 37 degrees C. How much this is an underestimate depends upon how much hydrophobic binding is greater with the uncharged compound.

Inhibition of L-lactate transport and band 3-mediated anion transport in erythrocytes by the novel stilbenedisulphonate N,N,N',N'-tetrabenzyl-4,4'-diaminostilbene-2,2'-disulpho nat e (TBenzDS).

(1) The synthesis of the novel stilbenedisulphonate N,N,N',N'-tetrabenzyl- 4,4'-diaminostilbene-2,2'-disulphonate (TBenzDS) is described, and its interaction with the lactate transporter and band 3 protein of erythrocytes investigated. At 10% haematocrit the IC50 (concn. required for 50% inhibition) for inhibition of transport of 0.5 mM L-lactate into rat erythrocytes at 7 degrees C was approx. 1.6 microM, as low as any other inhibitor of the transporter. In human erythrocytes at 10% haematocrit the IC50 value was increased from approx. 3 microM to 9 microM upon raising the temperature from 7 degrees C to 25 degrees C. (2) TBenzDS inhibited transport of L-lactate into rat erythrocytes in a manner that was competitive with the substrate, as is the case for some other stilbene disulphonate derivatives (Poole, R.C. and Halestrap, A.P. (1991) Biochem. J. 275, 307-312). (3) Increasing the haematocrit from 5 to 20% caused a 3-fold increase in the IC50 value for inhibition of L-lactate transport in rat erythrocytes. (4) TBenzDS was found to bind to erythrocyte membranes, with a partition coefficient (Pm) of 6000-7000 under all conditions tested. (5) TBenzDS also inhibited band 3-mediated sulphate transport in rat erythrocytes; 50% inhibition required approx. 2.5 microM TBenzDS for cells at 10% haematocrit. (6) TBenzDS is fluorescent, and an enhancement of this fluorescence occurs upon addition of BSA or erythrocyte membranes. The fluorescence enhancement caused by erythrocyte membranes is due to binding of the inhibitor to the band 3 protein at the same site as the stilbenedisulphonate 4,4'-diisothiocyanodihydrostilbene-2,2'-disulphonate (H2DIDS).

Effects of chain-length and unsaturation on affinity and selectivity at muscarinic receptors.

1. Lengthening the chain in diphenylacetylcholine decreases affinity for muscarinic cholinoceptors in guinea-pig ileum. Diphenylacetoxypropyldimethylamine and its quaternary trimethylammonium salt are roughly equiactive: the dimethylamine and the piperidine have some selectivity for ileum compared with atria, but are not as active nor as selective as 4-diphenylacetoxy-N-methylpiperidine (4-DAMP) methobromide (MeBr). With the weaker diphenylacetoxybutyl compounds the base is more active than the quaternary salt. 2. The diphenylacetoxybutyl-, cis-butenyl and trans-butenyl compounds have similar affinities. The quaternary salts are less active than the tertiary bases, but they are less selective than the butynyl analogues studied in earlier work. 3. 1,1-Diphenyl-1-hydroxy-2,4-hexadiynyl dimethylamine and its trimethylammonium salt are inactive in concentrations below 100 microM, as are the (+)-camphor-sulphonyl ester of 4-hydroxy-N-methyl piperidine and its methiodide. The (+/-)-phenylcyclopentylacetyl ester of 4-hydroxy-N-methylpiperidine methobromide is more active than its cyclohexyl analogue and than 4-DAMP MeBr but it is less selective than 4-DAMP MeBr. 4. The high selectivity of p-fluoro-hexahydrosila-diphenidol is confirmed but this compound has relatively low affinity (for ileum log K = 7.8). 5. The results indicate steric constraints to binding at muscarinic receptors which could be used to check molecular modelling of the receptor based on its known amino acid sequence. The group binding the charged nitrogen is probably at the mouth of a cavity which can accommodate two large rings (as in 4-DAMP MeBr) but with a depth less than about 7 A so that the rod-like hexadiynes cannot fit. Differences between types of receptor may only involve small changes in geometry secondary to differences in amino acids not directly involved in binding and the production of selectivity depends upon finding substituents which interfere with binding more at one type of receptor than at another.

The catalytic consequences of experimental evolution. Transition-state structure during catalysis by the evolved beta-galactosidases of Escherichia coli (ebg enzymes) changed by a single mutational event.

1. The first chemical step in the hydrolysis of galactosylpyridinium ions by the evolvant ebg enzyme is less sensitive to leaving-group acidity than in the case of the wild-type ebg enzyme, implying less glycone-aglycone-bond fission at the transition state. 2. The first chemical step in the hydrolysis of aryl galactosides by ebg enzyme is probably less sensitive to leaving-group acidity than in the case of ebg enzyme, possibly as a consequence of resulting in more effective proton donation to the leaving aglycone. 3. alpha-Deuterium kinetic isotope effects of 1.1(0) and beta-deuterium kinetic isotope effects of 1.0(0) were measured for the hydrolysis of galactosyl-enzyme intermediates derived from ebg and ebg enzymes: these effects are not compatible with reaction of the sugar ring through a 4C1-like conformation, or with an ionic glycosyl-enzyme intermediate. 4. The variation with pH of steady-state kinetic parameters for hydrolysis of p-nitrophenyl galactoside by ebg and ebg enzymes and of 3-methylphenyl beta-galactoside, 3,4-dinitrophenyl beta-galactoside and beta-galactosyl-3-bromopyridinium ion by ebg enzyme was measured. The steep, non-classical, fall in activity against p-nitrophenyl galactoside at low pH observed with ebg and ebg enzymes is not observed with ebg enzymes.