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Rat Brain Adenosine Deaminase After 2′‐Deoxycoformycin Administration: Biochemical Properties and Evidence for Reduced Enzyme Levels Detected by 2′‐[ 3 H]Deoxycoformycin Ligand Binding
Author(s) -
Padua R. A.,
Geiger J. D.,
Delaney S. M.,
Nagy J. I.
Publication year - 1992
Publication title -
journal of neurochemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.75
H-Index - 229
eISSN - 1471-4159
pISSN - 0022-3042
DOI - 10.1111/j.1471-4159.1992.tb09739.x
Subject(s) - adenosine deaminase , deoxycoformycin , adenosine , chemistry , in vivo , enzyme , endocrinology , medicine , biochemistry , biology , microbiology and biotechnology
Near total inhibition of brain adenosine deaminase (ADA) activity in rats injected with the potent ADA inhibitor 2′‐deoxycoformycin (DCF) was previously shown to reduce enzyme activity for up to 50 days during which time the enzyme exhibited reduced sensitivity to in vivo inhibition by DCF. Here, we investigated the biochemical properties of ADA and the basis for its reduced activity after DCF treatment. It was found that much higher doses of DCF were required to inhibit ADA in DCF‐treated compared with drug‐naive animals. Fourteen days after DCF administration, reduced ADA activity in brain homogenates was due to a decrease in V max , rather than to an altered K m of ADA for adenosine. DCF treatment had no effect on K i values for erythro ‐9‐(2‐hydroxy‐3‐nonyl)adenine inhibition of ADA. The IC 50 value for DCF inhibition of ADA in hypothalamus was unchanged. However, the K i for DCF inhibition of ADA in whole brain increased by fivefold. Sucrose gradient analysis of brain ADA revealed only one corresponding peak of activity and [ 3 H]DCF‐labeled ADA in DCF‐treated and control rats. A radioligand filtration assay with [ 3 H]DCF was developed to assess the effects of DCF on ADA protein levels. Over a roughly 200‐fold range of ADA activities the binding of [ 3 H]DCF was highly correlated with deaminase activity ( r = 0.99). In brain tissues taken 8 and 33 days after treatment of rats with DCF, [ 3 H]DCF binding was reduced to 27% and 48% of control levels, respectively. The results suggest that (1) although the biochemical properties of ADA are largely unchanged after treatment of rats with DCF, the enzyme in some brain regions has become less sensitive to this drug and (2) the prolonged reduction in ADA activity after DCF treatment is due to decreased levels of ADA protein possibly involving altered rates of degradation or production.