Kinetics of hydrolysis of aqueous dispersions of arsono-, sulfo-, phosphono- and phospholipids by phospholipase A2 from pig pancreas are characterized in terms of interfacial rate and equilibrium parameters. The enzyme with or without calcium binds with high affinity to the aqueous dispersions of the four classes of anionic lipids and shows the same general kinetic behavior. The rate of hydrolysis of anionic substrates does not show an anomalous change at the critical micelle concentration because the enzyme is present in aggregates even when bulk of the substrate is dispersed as a solitary monomer. Apparent affinities of the enzyme for the interface of different anionic lipids are virtually the same. Also, affinities of these substrates for the active site of the enzyme at the interface are comparable. However, a significant change in the catalytic turnover rate is seen as the sn-3 phosphodiester group is modified; the apparent maximum rate at saturating bulk substrate concentration, V(M)app values, increase in the order: homo- and arsonolipids < sulfo- < phosphono- < phospholipids. Not only the basis for the sn-2 enantiomeric selectivity but also the decrease in the rate of hydrolysis with the increasing chain length is due to a decrease in the value of V(M)app. Results show that even when the bulk concentration of anionic phospholipid is below cmc, hydrolysis occurs in aggregates of enzyme and substrate where the chemical step of the turnover cycle remains rate-limiting, which provides a basis for the assumption that V(M)app is directly related to Kcat. The fact that Kcat depends on the nature of the head group (phosphate, phosphonate, sulfate, arsonate) implies that the head group plays a critical role in the rate-limiting chemical step of the catalytic cycle, possibly during the decomposition of the tetrahedral intermediate. The significance of these results for the microscopic steady-state condition for hydrolysis at the micellar interface, mechanism of esterolysis by phospholipase A2, and inhibitor design are discussed.
The behaviour of Bunsen's cacodyl disulfide, Me 2 As(S)-S-AsMe 2 towards Lewis bases of group 15 of the Periodic Table was studied mainly by 1 H NMR. While Ph 3 N did not react, 4dimethylaminopyridine and triethylamine isomerized the disulfide to Me 2 AsSSAsMe 2 . Ph 3 P, (PhO) 3 P, (MeO) 3 P, (EtO) 3 P and Ph 3 As desulfurized the disulfide to Me 2 AsSAsMe 2 . (PhS) 3 P also desulfurized the disulfide but Me 2 As-S-SPh was also produced. Me 2 As-S-SPh was the main product with (PhS) 3 As nucleophile. In most sys-2077 tems, cationic and anionic species were formed. The results are best interpreted by initial attack of the nucleophile on the electrophilic AsϭS sulfur.
The reaction of Bunsen's cacodyl disulfide, Me2As(S)‐S‐AsMe2, with heavy metal cations in methanol produces insoluble salts (complexes) of dimethyldithioarsinic acid, Me2AsS2H, and dimethyl arsenium ion, Me2As:+. This arsenium ion prefers to react with Me2As(S)‐S‐AsMe2, when in excess, compared to AcO− or MeOH/H2O and it is also reactive towards sulfur (Sx, x = 1‐8) producing the stabilized dimethylarsino sulfenium cation, $\rm Me_{2}As- {\mathop{\rm{S}}\limits^{.}_{\ddot{}}} ^{+} \longleftrightarrow Me_{2}\mathop{\rm{A}}\limits^{+}_{} s=\mathop{\rm{S}}\limits^{.}_{\ddot{}}$. The complexes (Me2AsS2)xM (x = 1 or 2) are unstable in the presence of their own heavy metal cations decomposing to colored solids. In an attempt to prepare salts of Me2AsSH, the reactions of (Me2AsS2)xM with triphenylphosphine and trimethyl phosphite gave the metal sulfide and Me2As‐S‐AsMe2 instead.
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