Catalyzes the removal of methylamine from 4-methylaminobutanoate with the formation of succinate semialdehyde. Is involved in the catabolism of 4-methylaminobutanoate produced from nicotine. Has a very weak monoamine oxidase activity with 4-aminobutanoate. Cannot use spermidine, spermine, sarcosine, dimethylglycine, glycine, choline, betaine, alpha-methylamino isobutyrate, methylamine propionitrile and methylamino propylamine as substrate. 4-(methylamino)butanoate + H2O + O2 = H2O2 + methylamine + succinate semialdehyde kcat is 1230 sec(-1) with 4-methylaminobutanoate as substrate. kcat is 878 sec(-1) with 4-aminobutanoate as substrate. Optimum pH is 9.8. Alkaloid degradation; nicotine degradation. Monomer. Is transcribed only in the presence of nicotine. Belongs to the flavin monoamine oxidase family.