NAD(+) and NADP(+) were retained in continuously operated enzyme membrane reactors by nanofiltration membranes with retention rates between 0.86-0.98. For lower molecular weight substrates and products, retention rates below 0.35 were found. For the reductive amination of trimethylpyruvate to L-tert-leucine, the reactor was stably operated for IO days. The total turnover number could be increased 3.4 times to values higher than 7,900 by this approach.