5. ONIUM COMPOUNDS
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that now the purified enzyme sediments essentially as a homogeneous
protein, with a sedimentation constant of 8.2 S (20 ). These data have been
interpreted as indicating that the purified enzyme is essentially homogeneous but that it is not monodisperse because in solution it exists in
a partially polymerized state as a mixture of different molecular species
FIG. 15B
representing the monomer, dimer, trimer, and higher polymers respectively. One feature of this phenomenon that is particularly intriguing
is the fact that one of the substrates of the enzyme, homocysteine, is itself a sulfhydryl compound and as such capable of depolymerizing the
enzyme. However, all the attempts to show a relationship between the
catalytic activity of the protein and the polymerization reaction have
led to the conclusion that the polymerization reaction does not involve
a site, or sites, of the protein molecule which is, or are, active in the
enzymatic catalysis.
As to the nature of the bonds linking the monomer units in the polymerized enzyme it has been suggested, on the basis of the data presently
available, that the phenomenon depends upon the formation, and cleav-