METAL CHELATES IN THE STORAGE AND TRANSPORT OF NEUROTRANSMITTERS: FORMATION OF MIXED LIGAND CHELATES OF Mg²⁺‐ATP WITH BIOGENIC AMINES^1,2

Abstract

Abstract— Quantitative studies on the interactions of adenosine‐triphosphate and several biogenic amines with magnesium ion have been carried out in an attempt to correlate the thermodynamic stabilities of the metal‐binding of the amines with the in vivo affinities of the amines for granule‐binding. Equilibrium data indicate that in each of the ternary chelate systems (viz. Mg2+‐ATP‐amine), the predominant reaction in the pH range 3.0–7.0 is the formation of a magnesium‐ATP chelate with a stability constant, log KML=3.22 ± 0.02. Each of the biogenic amines coordinates with Mg2+‐ATP system in the pH range 7.0–10.5 to form the mixed ligand chelate (or ternary chelate), Mg2+‐ATP‐amine(1:1:1). The stability constants for the binding of the amines with Mg2+‐ATP are: (i) norepinephrine (NE) = 2.34 ± 0.32; (ii) epinephrine (E) = 2.95 ± 0.08; (iii) dopamine (DA) = 3.05 ± 0.06; (iv) octopamine (OA) = 1.93 ± 0.12; (v) 6‐hydroxydopamine (6‐HDA) = 2.42 ± 0.14; (vi) 3‐methoxynorephedrine (MeN) =2.76 ± 0.09; (vii) amphetamine (AA) =2.09 ± 0.05; (viii) tyramine (TA) = 2.60 ± 0.04; (ix) phenylethylamine (PEA) = 0. A general correlation is indicated between the stability constants (binding strengths) of the amine chelates and the metal‐binding functionalities of the amines on the one hand and their vesicular binding characteristics in in vivo systems on the other (Carlsson and Waldeck, 1966). The Mg2+‐ATP‐dependant amine storage mechanism of KIRSHNER (1962a;b) and Carlsson, Hillårp and Waldeck (1963) is discussed both in the light of the data on metal chelate stability and of a significant modification of metal coordination hypothesis.

Nmr studies of catecholamines. Acid dissociation equilibria in aqueous solutions

“…A more appropriate equilibrium model which takes into account the possibility of competition between the ring and the chain deprotonations is given by [15] (1) can also be applied for the analysis of the shift data of the other compounds, but substituting for f, fR = fz + fA and fs = fN + fA for the fitting of the ring and the side-chain data respectively. Once the microconstants were derived, the macroscopic acid dissociation constants, usually obtained by potentiometric titration, can be calculated by the relations Kl = D(Z + N)/C = kl + kz K~ -1 = (Z + N)/AD = k31 +kg 1 (6) The ring and the side chain proton shift of each compound were analyzed simultaneously following the procedure described above, with the same three microconstants as fitting parameters. The values obtained for the acidity micro-and macroconstants thus obtained are summarized in table 3.…”

“…spectrometry methods, Lewis [I], Reigelman et al [2] and Kappe and Armstrong [3] assigned the first acid dissociation constant to the ionization of the phenolic group ' and the second to ammonium deprotonation. In more recent studies however, Jameson and Neillie [4] and Rajan et al [5,6] introduced an opposite assignment, whereas Martin [7] and Antikainen and Witikainen [8] again assigned the phenolic group as the more acidic. It should also be noted that in studies of complexation of phenolic amines it has commonly been assumed (erroneously, as will be shown below) that the ammonium group is the stronger acid (e.g.…”

“…It should also be noted that in studies of complexation of phenolic amines it has commonly been assumed (erroneously, as will be shown below) that the ammonium group is the stronger acid (e.g. [5,6,[9][10][11][12] and references therein).…”

Chemical evidence that catecholamines are transported across the chromaffin granule membrane as noncationic species.

“…In view of our data, it is also possible that variation in external pH affected membrane functions, because the pH dependence for transport titrates with a pKa of approximately 6.4 (28), whereas norepinephrine titrates at higher pK. values (8.57, 9.73, 11.13) (13). In addition, the rate of interconversion of catecholamine species may be rapid compared to the rate of binding or transport.…”

“…The wavy line on isoproterenol in m denotes the bond broken when the first proton dissociates. For norepinephrine (which has no methyl groups on the nitrogen atom) the pK is 8.57, and for epinephrine (one methyl group) the pK is 8.73 (12,13). structure, this compound exists only as the positively charged species.…”