I think the studies show that there could be such a thing as a PQQ deficiency (PQQ deficiency affects mitochondrial function in mice). In that sense it might be a vitamin.
Although note the comments of Chris Anthony DSc. Emeritus Professor of Biochemistry (School of Biological Sciences, University of Southampton):
"No mammalian PQQ-containing enzyme (quinoprotein) has been described. If such an enzyme does exist then PQQ would almost certainly be a vitamin [analogous to riboflavin, needed in the diet for production of essential flavoproteins]. Such an enzyme was reported in Nature in 2003, leading to the claim for the discovery of the first new vitamin for 55 years. This claim was based on some basic misunderstandings of enzymes, protein structure and databases. We have been involved in refuting this claim."
"..their claim was based on sequence analysis of an enzyme, predicted to be involved in mouse lysine metabolism, which showed that part of the protein has a ‘propeller fold', which is a feature of all PQQ-dependent dehydrogenases. What the evidence actually suggests is that their (predicted) protein is an interesting novel protein part of which has a propeller structure; but there is no evidence that it is a PQQ-dependent dehydrogenase. In essence their argument is this: Our mouse protein has a ‘propeller’ structure [true]; All PQQ-containing proteins have a propeller structure [true]; Therefore our protein contains PQQ [false]. This is the equivalent of the well known false argument: All trees are green; My coat is green; My coat is a tree.
"When I pointed out to the journal Nature that their high reputation was being used to justify investments of millions of dollars in the development of PQQ as a vitamin, they investigated the original paper, agreed with our objections and published our argument against it (Felton & Anthony, Nature Vol. 433, 2005). They also published (alongside ours) a paper by Rucker disagreeing with the conclusions of Kasahara and Kato on nutritional grounds, concluding “that insufficient information is available so far to state that PQQ uniquely performs an essential vitamin function in animals”.
It should also be noted that the enzyme which Kasahara and Kato claim to be involved in lysine breakdown does not occur in mice; it is an irreversible enzyme involved in the opposite process of lysine biosynthesis in yeast. The whole of this part of their paper is biochemical nonsense."
Edited by hamishm00, 27 September 2012 - 01:36 PM.