During detailed restriction enzyme mapping of the human X‐linked gene Gd, specifying the enzyme glucose 6‐phosphate dehydrogenase (G6PD), we have observed the presence, over a 14‐kb DNA region spanning across the 3′ end of the G6PD transcript, of a large number of methylatable sites. These include 60 HpaII sites, 13 SmaI sites, 22 AvaI sites and 46 HhaI sites. In male leukocyte DNA the majority of HpaII sites are resistant to digestion, indicating that they are in the Cm5CGG form. However, a few sites are found reproducibly unmethylated in 24 samples analyzed. By double and triple digestions we have mapped five unmethylated sites, four of which are within the gene transcript and one distal to the end of transcription. We have also identified a number of sites which are fully methylated, whereas for others the methylation status could not be positively assessed. Thus, in a housekeeping gene expressed in leukocytes, the 3′ end is extensively methylated, but some specific sites are unmethylated. In female leukocyte DNA, we found that all sites methylated in males were also methylated. However, of the five sites that are unmethylated in males two are partly methylated in females. This additional site‐specific methylation involves approximately 50% of the female leukocyte DNA, and we show evidence that it is associated with the inactive X‐chromosome.
A kinetic model is developed for the ozonation of
oxalic acid catalyzed by solid MnO2. The
rate
of ozonation is limited by the adsorption of oxalic acid on the
catalyst surface and by the
deactivation of a fraction of the active sites, because of an
irreversible reaction with ozone.
Moreover, the model includes the ozonation of oxalic acid
catalyzed by dissolved manganese.
This kinetic model allows for a fairly good correlation of the
experimental data when the average
size of the catalyst particles is smaller than about 10 μm. When
larger particles are employed,
noticeable mass-transfer limitations are encountered, mainly deriving
from the diffusion of both
reactants from the liquid bulk to the solid surface (external and
internal). The dependence of
the rate constant on pH is experimentally determined and explained in
terms of the changing
chemical structure of the active sites and of the dissociation
equilibrium of oxalic acid.
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