To put this number in some perspective, if the same constraint on composition was applied to natural peptides, there would be only 66 possible molecules (one distinct tripeptide, 46 distinct dipeptides and 19 single amino acids). It would be interesting to push this toy analysis a little further - what fraction of available (13 atom reasonable) chemical space do natural human metabolites occupy, are there any interesting patterns, etc.?
This database is really exciting.
%A L.C. Blum %A J.-L. Reymond %J J. Am. Chem. Soc. %D 2009 %V 131 %N (25) %P 8732-8733 %O DOI:10.1021/ja902302h