Kinetic Properties of Myrosinase in Hydrated Reverse Micelles

Abstract

AbstractThe enzymatic activity of myrosinase (thioglucoside glucohydrolase, EC 3.2.3.1) solubilized in reverse micelles, which were formed in isooctane by bis(2‐ethylhexyl) sodium sulfosuccinate (AOT), was studied using sinigrin (allyl glucosinolate) and glucotropeolin (benzyl glucosinolate) as substrates. The enzyme in reverse micelles follows Michaelis‐Menten kinetics for both substrates, showing similar kinetic values, whereas in water solution sinigrin seems to be the better substrate. The effect of the water concentration, W0 (W0=[H20]/[AOT]), on the steady‐state velocity of the myrosinase does not induce the bell‐shaped activity profile observed for many other enzymes. The optimal W0 for both substrates appears to be widespread between 10 and 40. The pH profile of myrosinase activity in reverse micelles shows an increase of activity from pH 4 to pH 6. Above this value and up to pH 9, the activity remains constant, establishing a broad plateau at its maximum. It was demonstrated that myrosinase activity in reverse micelles, as well as in water solution, strongly depends on the temperature, showing an extraordinary maximum of activity with both substrates around 60 °C. As in water, the enzyme appears to be strongly activated by ascorbate in reverse micelles with both sinigrin and glucotropeolin. All the above properties of the enzyme in reverse micelles, and in particular (i) the possibility of varying the water concentration and the pH within relatively wide ranges, (ii) the high thermal stability of the enzyme together with the exceptionally high optimum reaction temperature, and (iii) the activation of the enzyme by ascorbate more than in water solutions, strongly suggest the use of this enzyme solubilized in reverse micelles to good advantage for producing biologically interesting nitrogen‐and sulfur‐containing compounds from glucosinolate hydrolysis.