The announcement by Kasahara and Kato of pyrroloquinoline quinone (PQQ) as a 'new' vitamin has received considerable attention. We have since attempted to reproduce the findings on which their conclusion is based, namely that defects in lysine metabolism occur in PQQ-deprived rodents. However, we find that the activity of alpha-aminoadipic acid-delta-semialdehyde (AAS) dehydrogenase in liver and plasma levels of alpha-aminoadipic acid (AAA), both of which act as indicators of lysine degradation in mammals, are not affected by changes in PQQ dietary status. Our results call into question the identification of PQQ as a new vitamin.