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LETTERS TO THE EDITOR
With respect to PG, Dr. Katz points out that this acid-insoluble form of glycogen (PG) is not a "discrete species". We can find no place in our paper (8) in which we suggested that this is so; in fact, we suggested the opposite. In our previous work (1, 7), we proposed that PG is one of two fractions of the glycogen granules and that they exist as a continuum. Our electron microscopy work (5) shows this conclusively, and our review (6) makes the point very clearly. We believe that the work from Whalen’s laboratory [Lomako et al. (3)] cited by Dr. Katz was pioneering and insightful; we also believe that the results reported in those studies are open to alternative interpretations. As noted above, we questioned this finding some years ago and did research to resolve it. It should be noted that Dr. Whalen’s group recently wrote (4) that their investigation (3) in 1991 "estimated, very roughly [emphasis mine], that the Mr of this TCA-soluble glycogen ... was 400 kDa." Nowhere in our recent work (8) do we state or imply that PG is a separate species of glycogen. It appears that Dr. Katz has misinterpreted our discussion in this paper. Our previously published work (5), which he does not cite, demonstrates very clearly that the granules are a continuum, ranging in diameter from approximately 8 to 42 nm, and we state strongly that PG cannot be a discrete species of glycogen granule.
Dr. Katz’s other issue involves one of our three main measurements, that of the in vitro activity of glycogenin. He points out that we have used a published method that is different from that of his work (2). It is difficult to compare our results with those of his study, as it appears that the muscle samples used in his study may not be comparable to those in our investigation on the basis of sampling sites and duration of storage.
We agree with Dr. Katz that our description in the discussion that the activity assay measures the "self-glycosylating ability of GN-1" (glycogenin) is misleading. Rather, we should have reiterated that our activity assay measures the transglycosylation of n-dodecyl-
-D-maltoside by glycogenin, which is clearly stated in our MATERIALS AND METHODS section. The "activity" assay was used as an indirect measurement of glycogenin protein rather than for use in the classical sense.
Dr. Katz states that we do not report results from control experiments demonstrating that the activity measured was solely attributable to glycogenin. In our present (unpublished) work, the large majority of the transglycosylation appears to occur following amylase treatment of the sample, which would support the reaction’s occurring via glycogenin. Of course, we cannot rule out the possibility that other enzymes may contribute; however, their contributions appear to be minimal (<10 mU·mg protein–1·min–1).
REFERENCES
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