UDP-glucose:(6-methoxy)podophyllotoxin 7- O-glucosyltransferase from suspension cultures of Linum nodiflorum

Abstract

Cell cultures of Linum species store 6-methoxypodophyllotoxin (MPTOX), podophyllotoxin (PTOX) and related lignans as O-glucosides. UDP-glucose:(M)PTOX 7- O-glucosyltransferase has been detected and characterised in protein preparations of suspension-cultured cells of Linum nodiflorum L. (Linaceae). The maximal lignan glucoside contents in the cells are preceded by a rapid increase of the specific glucosyltransferase activity on day six of the culture period. MPTOX glucoside is the major lignan with up to 1.18 mg g −1 of the cell dry wt which is more than 30-fold of the PTOX glucoside content. Of the three aryltetralin lignans tested as substrates, PTOX and MPTOX display comparable apparent K m values of 4.7 and 5.4 μM, respectively. 5′-Demethoxy-6-methoxypodophyllotoxin is converted with the highest velocity of 25.2 pkat mg −1 while also possessing a higher K m of 14.7 μM. Two-substrate test series indicate that all three compounds compete for the active site of a single protein. The structurally similar lignan β-peltatin acts as competitive inhibitor as well. However, the 6- O-glucosidation is most likely catalysed by a separate enzyme. The (M)PTOX 7- O-glucosyltransferase works best at a pH around 9 and a temperature around 35 °C. A 15–30% increase of the reaction rate is effected by the addition of 0.9 mM Mn 2+.