REGULATION OF METHIONINE ADENOSYLTRANSFERASE CATALYTIC ACTIVITY AND MESSENGER-RNA IN SH-SY5Y HUMAN NEUROBLASTOMA-CELLS

The human neuroblastoma cell line SH-SY5Y was used to study the regula tion of methionine adenosyltransferase (MAT II; E.C.2.5.1.6.) catalyti c activity and transcript levels in cells of neuronal origin. The cell s were exposed for 24 hr to a medium containing different concentratio ns of methionine (MAT substrate) as well as medium deficient of methio nine. Furthermore, cells were treated with hydroxycobalamin, SAM, and the competitive MAT inhibitor cycloleucine. The MAT catalytic activity was inversely correlated to methionine concentrations, e.g. MAT V-max increased 2-fold in cells grown in methionine-deficient medium as com pared with cells cultured under standard conditions. Interestingly, MA T K-m also increased from 9.04 +/- 0.44 to 12.08 +/- 0.83 in the methi onine-deficient medium. Hydroxycobalamin caused an increase in activit y at 40 mu M while a decrease was observed at higher concentrations (1 00, 200, and 400 mu M). Cycloleucine caused a significant inhibition o f MAT catalytic activity, i.e. the inhibition was approximately 50% in the presence of 4 mM cycloleucine. The relevance of these results for the understanding of observations on MAT catalytic activity in brains of patients with Alzheimer's disease is discussed. (C) 1998 Elsevier Science Inc.