IN VITRO CONVERSION OF BILE PIGMENTS TO UROBILINOIDS BY A RAT CLOSTRIDIA SPECIES AS COMPARED WITH HUMAN FECAL FLORA .2. MESOBILIRUBIN AND DIHYDROMESOBILIRUBIN

Mesobilirubin or dihydromesobilirubin were converted by a pure strain of rat Clostridia to class II and III urobilinoid chromogens (l-half-stercobilinogen and l-stercobilinogen). This was also true in some experiments with various samples of human fecal flora; in others, only or mainly class I (d-urobilinogen) was formed. Mass spectral analysis revealed that with mesobilirubin as substrate, the corresponding urobilin was the diethyl (H42, mol wt 590) form while with bilirubin the less saturated, presumably monovinyl (mol wt 588) type, was also noted. In a number of experiments with fecal flora, mixtures of class I and II were formed as determined by FeCl3 oxidation patterns, optical activity, and mass spectra. In certain of these experiments in which mesobilirubin was the substrate, dihydromesobilirubin was also identified and its importance as a branch point intermediary is discussed. Various evidence is considered pointing to a new concept according to which d-urobilinogen H44, in contrast to its l-enantiomer, must be uncoiled and recoiled, in transition to l-half-stercobilinogen. This process requires extra energy of activation which readily accounts for the appearance of d-U (class I), either in bacterial cultures as presently studied, or in some samples of feces in vivo. © 1969.