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Tyrosine Hydroxylase Regulation: Apparent Kinetic Alterations Following Incubation of Brain Slices in a Sodium‐free Medium
Author(s) -
Bustos Gonzalo,
Simon Jay,
Roth Robert H.
Publication year - 1980
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.1471-4159.1980.tb12488.x
Subject(s) - tyrosine hydroxylase , incubation , chemistry , enzyme , enzyme assay , biochemistry , dopamine , choline , tyrosine , sodium , sucrose , tyrosine 3 monooxygenase , endocrinology , biology , organic chemistry
In an attempt to determine if alterations in intraneuronal Ca 2+ may regulate tyrosine hydroxylase activity, brain slices were subjected to experimental manipulations known to increase the intraneuronal concentration of free Ca 2+ ions. Incubation of either striatal or olfactory tubercle slices in a Na + ‐free medium for 15 min at 37° resulted in a marked increase in the activity of tyrosine hydroxylase present in the 20,000 g supernatant fraction of homog‐enates prepared from the slices. Tyrosine hydroxylase isolated from slices previously incubated in a Na + ‐free, choline‐enriched medium or in a Na + ‐free, sucrose‐enriched medium exhibited maximal activities when assayed at pH 6.0 and 7.0, respectively. However, the percentage stimulation of enzyme activity induced by incubation of the slices in a Na + ‐free medium was maximal when the enzyme assays were performed at pH 7.0. The observed increase in enzyme activity seems to be mediated by a decrease in the apparent K m of the enzyme for pteridine cofactor, regardless of whether the kinetic enzyme analyses were conducted at pH 6.0 or 7.0, and by an increase in the K 1 of the enzyme for end‐product inhibitor dopamine. The apparent kinetic changes in the enzyme do not seem to result from alterations in the endogenous dopamine content of the slices, and they are independent of any increase in dopamine release that might have occurred as a response to the augmented intraneuronal Ca 2+ concentration. Furthermore, the activation of tyrosine hydroxylase produced by incubating slices in a Na + ‐free medium is observed even in slices depleted of dopamine by pretreatment of rats with reserpine 90 min before preparation of brain slices. The activation of tyrosine hydroxylase observed under these experimental conditions does not seem to be mediated by cAMP or by a cAMP‐dependent phosphorylation process. It is suggested that the changes in tyrosine hydroxylase reported are mediated primarily by a rise in the free Ca 2+ concentration within the nerve tissue. These observations are consistent with the hypothesis that the kinetic activation of tyrosine hydroxylase produced after depolarization of central dopaminergic neurons may occur through a Ca 2+ ‐dependent event other than transmitter release.