Intramolecular group transfer is a characteristic of neurotoxic esterase and is independent of the tissue source of the enzyme. A comparison of the aging behaviour of di-isopropyl phosphorofluoridate-labelled proteins in brain, spinal cord, liver, kidney and spleen from hen and in human placenta

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Abstract
Neurotoxic esterase activity was measured in homogenates of human placenta and hen brain, spinal cord, liver, kidney and spleen. The activity in liver comprised less than 20% of the Paraoxon-resistant esterases, but in the other tissues neurotoxic esterase accounted for over 50%. The same tissues were labelled with [3H]di-isopropyl phosphorofluoridate, and any isopropyl group transferred on to protein during ‘aging’ of the labelled enzymes (alkali-volatilizable tritium) was measured. No Paraoxon-sensitive labelled sites were found to age in this way in any tissue. In brain, the Paraoxon-resistant alkali-volatilizable-tritium-labelled sites correlated with the number of neurotoxic esterase labelled sites, indicating that ‘aging’ and isopropyl group transfer were 100% efficient. The site receiving the transferred isopropyl group was characterized by analysing the distribution of radiolabelled proteins on gel-filtration chromatography in the presence of SDS. In particulate preparations from each tissue, the protein-bound alkali-volatilizable tritium (transferred isopropyl group) was attached to a polypeptide of Mr 178 000. This same polypeptide also bore the isopropyl-phosphoryl group of neurotoxic esterase, indicating that aging of neurotoxic esterase is an intramolecular group transfer. The apparent turnover number for the enzyme (average 1.6×10(5) min-1) was approximately the same in each hen tissue, confirming that closely similar enzymes were present in brain, spinal cord, liver and spleen. The apparent turnover for the human enzyme was 1.8-fold higher than that for the hen enzyme. The concentration of the neurotoxic esterase phosphorylated subunit in brain, spinal cord, spleen, placenta and liver was 14.6, 3.8, 7.4, 3.3 and 3.8 pmol/g of tissue. The evidence indicated that neurotoxic esterase is present in each tissue except kidney, and that isopropyl group transfer on ‘aging’ occurs on this enzyme only. This process is an intramolecular transfer of the group within the same polypeptide.