I had been looking for evidence of proton transfer in enzymes that catalyzed aldose-ketose interconversions.    From Wordnik.com.  [Irwin Rose - Autobiography] Reference

As shown in Table 1, isomerases that are specific for 2R-aldoses were found to activate the 1-R proton of the ketose produced.    From Wordnik.com.  [Irwin Rose - Autobiography] Reference

If now the aldose groups tend to pass over into the starch form, representing a temporary overflow product of the assimilating energy, it would appear that the ketose or levulose groups are preferentially used up in the elaboration of the permanent tissue.    From Wordnik.com.  [Researches on Cellulose 1895-1900] Reference

The reason it is a 4 membered ring is because it is a ketose, as opposed to an aldose.    From Wordnik.com.  [ScienceBlogs Channel : Life Science] Reference

Aldose-ketose isomerases.    From Wordnik.com.  [Irwin Rose - Autobiography] Reference