Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: EC:2.7.7.48 (transcriptase)
 9,479 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Cholinergic agonists stimulate isotonic fluid secretion in the parotid gland. This process is driven by the apical exit of Cl-, which enters the cells partly via Cl-/HCO-3 exchange across the basolateral membrane. Acidification of the cytosol by the extrusion of HCO-3 is prevented by the concomitant activation of the Na+/H+ exchanger (NHE), which is directly activated by cholinergic stimulation. Multiple isoforms of the NHE have been described in mammalian cells, but the particular isoform(s) present in salivary glands and their mechanism of activation have not been defined. Reverse transcriptase-polymerase chain reaction with isoform-specific primers was used to establish that NHE-1 and NHE-2, but not NHE-3 or NHE-4, are expressed in parotid glands. The presence of NHE-1 was confirmed by immunoblotting and immunofluorescence, which additionally demonstrated that this isoform is abundant in the basolateral membrane of acinar cells. The predominant role of NHE-1 in carbachol-induced Na+/H+ exchange was established pharmacologically using HOE694, an inhibitor with differential potency toward the individual isoforms. Because muscarinic agonists induce stimulation of protein kinases in acinar cells, we assessed the role of phosphorylation in the activation of the antiport. Immunoprecipitation experiments revealed that, although NHE-1 was phosphorylated in the resting state, no further phosphorylation occurred upon treatment with carbachol. Similar phosphopeptide patterns were observed in control and carbachol-treated samples. Together, these findings indicate that NHE-1, the predominant isoform of the antiporter in the basolateral membrane of acinar cells, is activated during muscarinic stimulation by a phosphorylation-independent event. Other processes, such as association of Ca2+-calmodulin complexes to the cytosolic domain of the antiporter, may be responsible for the activation of Na+/H+ exchange.
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PMID:Muscarinic agonists induce phosphorylation-independent activation of the NHE-1 isoform of the Na+/H+ antiporter in salivary acinar cells. 899 60

The activity of melanosome-associated tyrosinase in human melanocytes differs based on racial skin type. In melanocytes from Black skin, tyrosinase activity is high while in White melanocytes the activity of the enzyme is low. Recent studies suggest that low tyrosinase activity in White melanocytes may be due to an acidic pH environment within the melanosome. Because sodium/hydrogen (Na(+)/H(+)) exchangers (NHEs) are known to regulate intracellular pH, melanocytes were treated with NHE inhibitors to determine what effect this inhibition might have on tyrosinase activity. Treatment of Black melanocytes with ethyl-isopropyl amiloride (EIPA) caused a rapid dose-dependent inhibition of tyrosinase activity. This inhibition was not due to either direct enzyme inhibition or to a decrease in tyrosinase abundance. In contrast, treatment of White melanocytes with EIPA, cimetidine, or clonidine resulted in little inhibition of tyrosinase activity. Reverse transcriptase-polymerase chain reaction (RT-PCR) and Western blot analysis showed that both Black and White melanocytes expressed mRNA and protein for NHE-1, NHE-3, NHE-5, NHE-6, and NHE-7. Immunohistochemical analysis showed that NHE-7 and NHE-3 co-localized with the melanosomal protein, Tyrosinase Related Protein-1 (TRP-1). In addition, the vesicular proton pump, vesicular ATPase (V-ATPase), was found to be present in both White and Black melanosomes, indicating that organelles from both racial skin types are capable of being acidified. The results suggest that one or more NHEs may help regulate melanosome pH and tyrosinase activity in human melanocytes.
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PMID:The relationship between Na(+)/H(+) exchanger expression and tyrosinase activity in human melanocytes. 1526 99