Butyrylcholinesterase in the visual ganglia of the squid todarodes sagittatus I. (cephalopoda). isolation, molecular forms, interaction with substrates and inhibitors

Abstract

1. Soluble butyrylcholinesterase (BuChE) was isolated from the visual ganglia of the squid Todarodes sagittatus L. Gel-chromatography on Sephadex G-200 columns resulted in its separation into three molecular forms.

2. The major component with a molecular mass of 180kDa was used for kinetic study.

3. The substrate analysis revealed squid enzyme to be BuChE of unusual type.

4. Unlike typical BuChE (EC 3.1.1.8), squid enzyme splits acetyl-β-methylcholine (AMCh) with a relatively high rate, alongside with common BuChE substrates—butyrylcholine (BCh), propionylcholine (PCh), acetylcholine (ACh), butyrylthiocholine (BTCh) and acetylthiocholine (ATCh), the enzymic hydrolysis being suppressed by excess of all these substrates.

5. Among them, the highest values of k_{cat and}k_cat/K_m were found for BCh and BTCh. Maximal activity of the enzyme was noticed at low BCh and BTCh concentrations (1–2 mM).

6. Tetraalkylammonium ions exhibit a mixed type of inhibition and suppress the substrate inhibition of squid BuChE.

7. Among organophosphorus inhibitors (OPI), the methylthiophosphonates are most potent for squid BuChE, and for some phosphates, selective OPI of typical BuChE, are potent as well.

8. By the pattern of selectivity to OPI, squid enzyme differs from both typical BuChE of horse serum and acetylcholinesterase (EC 3.1.1.7) from bovine erythrocytes.

9. Some details of the active center structure of squid BuChE compared to that of typical enzymes are discussed.