THE SPECIFICITY OF RAT BRAIN ACETYLCHOLINESTERASE FOR N-ALKYL ANALOGUES OF ACETYLCHOLINE

Substrate recognition of the membrane-associated sialidase NEU3 requires a hydrophobic aglycone

The human neuraminidases (NEU) consist of a family of four isoforms (NEU1-NEU4). Members of this enzyme family are proposed to have important roles in health and disease through regulation of the composition of cellular sialosides. The NEU3 isoform is a membrane-associated enzyme that cleaves glycolipid substrates. However, few reports have examined the substrate specificity of the enzyme for non-natural substrates. We report here a series of 11 synthetic trisaccharides that feature modifications of the aglycone or the Neu5Ac residue of an octyl β-sialyllactoside. The time course of substrate cleavage by NEU3 was monitored using an electrospray ionization mass spectrometry assay to obtain relative rates (k(rel)). We observed that NEU3 substrate activity was directly dependent upon the hydrophobicity of the aglycone but had no apparent requirement for features of the ceramide headgroup. We also observed that trisaccharides with incorporated azide groups in the Neu5Ac residue at either C9 or the N5-Ac position were substrates, and in the case of the N5-azidoacetyl derivative, the activity was superior to that of GM3. However, the incorporation of larger aryl groups was tolerated only at C9, but not at N5-Ac. We propose a two-site model for enzyme recognition, requiring interaction at both the Neu5Ac residue and the hydrophobic aglycone.

General occurrence of binding to acetylcholinesterase-substrate complex in noncompetitive inhibition and in inhibition by substrate

To assess the relative importance of binding to enzyme-substrate complex (E.S) and to acetylenzyme (EA), noncompetitive inhibition has been studied in hydrolysis by acetylcholinesterase (AcChE) of cationic and uncharged substrates - acetylcholine (AcCh), 3,3-dimethylbutyl acetate, n-butyl acetate, 2-(methylammonio)ethyl acetate, 2- (N,N-diethyl-N-n-butylammonio)ethyl acetate (DEBAAc) and 2-(methylsulfonyl)ethyl acetate. For the N-trimethyl quaternary ions related to AcCh, tetramethylammonium ion, choline and choline ethyl ether, noncompetitive inhibition (Ki(nonc) is more favorable with the slower substrates than with AcCh, i.e., when E.S greater than EA, and is attributed to formation of enzyme-substrate-inhibitor complexes, E.S.I'. Noncompetitive inhibition by tetraethyl-, tert-butyl- and isopropylammonium ions, and acetamidocholine and its lower dimethyl analogue, is also attributed to E.S.I' complexes. Peripheral binding of these inhibitors decreases acylation more than deacylation. Some tertiary dimethylamonio ions have more favorable Ki(nonc) values with AcCh, decreasing deacylation more than acylation. The substrate DEBAAc is a more effective noncompetitive than competitive inhibitor in hydrolysis of AcCh, indicating that it binds more strongly in a peripheral site than in the active site of the free enzyme. In its hydrolysis by AcChE, it acts as its own noncompetitive inhibitor, by this non-productive binding. Formation of E.S.I' complexes is a general characteristic of hydrolysis by AcChE and decrease in rates at high concentrations of AcCh and related substrates is attributed to peripheral regulatory site binding, formation of E.S.S' complexes, rather than to binding to the acetylenzyme.

Spontaneous release of acetylcholine and acetylhomocholine from mouse forebrain minces: cytoplasmic or vesicular origin

The objective of this study was to determine the subcellular origin of cholinergic transmitter released spontaneously from mouse forebrain minces. To accomplish this objective, minces were pretreated in ionic media and then loaded with [14C]homocholine, an analog of choline, to form the false transmitter [14C]acetylhomocholine [( 14C]AHCh). The ratio of the false transmitter [14C]AHCh to the true transmitter ACh was then used as an index of cholinergic transmitter contents for both the cytoplasmic (S3) and vesicle-bound (P3) fractions. Three different pretreatment procedures were used to cause the following changes in S3 and P3 false to true transmitter ratios prior to spontaneous release: 1) a small increase in the S3 ratio of [14C]AHCh to acetylcholine (ACh) and a large increase in the P3 ratio of [14C] AHCh to ACh; 2) a decrease in the S3 ratio of [14C]AHCh to ACh and an increase in the P3 ratio of [14C]AHCh to ACh; 3) an increase in the P3 ratio of [14C]AHCh to ACh without affecting the S3 ratio of [14C]AHCh to ACh. The influence of each pretreatment on these subcellular ratios was then compared with its influence on the spontaneous release ratio of [14C]AHCh to ACh. In all 3 instances, the influence of pretreatment on the ratio of spontaneously released false and true cholinergic transmitters from minces coincided with the effect of pretreatment on the pre-release ratio of false to true transmitter in the S3 fraction. These results suggest that much of the cholinergic transmitter which is spontaneously released from mouse forebrain occurs from the cytroplasmic fraction.