Localization and hormonal control of serine dehydratase during metabolic acidosis differ markedly from those of phosphoenolpyruvate carboxykinase in rat kidney

Acidosis-induced downregulation of hepatocyte mitochondrial aquaporin-8 and ureagenesis from ammonia

It has been proposed that, during metabolic acidosis, the liver downregulates mitochondrial ammonia detoxification via ureagenesis, a bicarbonate-consuming process. Since we previously demonstrated that hepatocyte mitochondrial aquaporin-8 channels (mtAQP8) facilitate the uptake of ammonia and its metabolism into urea, we studied whether mtAQP8 is involved in the liver adaptive response to acidosis. Primary cultured rat hepatocytes were adapted to acidosis by exposing them to culture medium at pH 7.0 for 40 h. Control cells were exposed to pH 7.4. Hepatocytes exposed to acid medium showed a decrease in mtAQP8 protein expression (-30%, p < 0.05). Ureagenesis from ammonia was assessed by incubating the cells with (15)N-labeled ammonia and measuring (15)N-labeled urea synthesis by nuclear magnetic resonance. Reduced ureagenesis was found in acidified hepatocytes (-31%, p < 0.05). In vivo studies in rats subjected to 7 days acidosis also showed decreased protein expression of hepatic mtAQP8 (-50%, p < 0.05) and reduced liver urea content (-35%; p < 0.05). In conclusion, our in vitro and in vivo data suggest that hepatic mtAQP8 expression is downregulated in acidosis, a mechanism that may contribute to decreased ureagenesis from ammonia in response to acidosis.

Development of a novel cysteine sulfinic Acid decarboxylase knockout mouse: dietary taurine reduces neonatal mortality

We engineered a CSAD KO mouse to investigate the physiological roles of taurine. The disruption of the CSAD gene was verified by Southern, Northern, and Western blotting. HPLC indicated an 83% decrease of taurine concentration in the plasma of CSAD^−/−. Although CSAD^−/− generation (G)1 and G2 survived, offspring from G2 CSAD^−/− had low brain and liver taurine concentrations and most died within 24 hrs of birth. Taurine concentrations in G3 CSAD^−/− born from G2 CSAD^−/− treated with taurine in the drinking water were restored and survival rates of G3 CSAD^−/− increased from 15% to 92%. The mRNA expression of CDO, ADO, and TauT was not different in CSAD^−/− compared to WT and CSAD mRNA was not expressed in CSAD^−/−. Expression of Gpx 1 and 3 was increased significantly in CSAD^−/− and restored to normal levels with taurine supplementation. Lactoferrin and the prolactin receptor were significantly decreased in CSAD^−/−. The prolactin receptor was restored with taurine supplementation. These data indicated that CSAD KO is a good model for studying the effects of taurine deficiency and its treatment with taurine supplementation.

Downregulation in the expression of the serine dehydratase in the rat liver during chronic metabolic acidosis

Blood pH controls the activity of important regulatory enzymes in the metabolism. Serine dehydratase (SerDH) transforms l-serine into pyruvate and ammonium and is involved in the regulation of gluconeogenesis from serine in the rat liver. In this work, we investigate the effect of chronic metabolic acidosis on the kinetics, specific protein level, tissue location, and mRNA levels of rat liver SerDH. Experimental acidosis was induced in rats by ingestion of 0.28 M ammonium chloride solution for 10 days. Acidosis significantly (P<0.05) decreased SerDH activity at all substrate concentrations assayed. Moreover, the Vmax value was 38.50+/-3.51 mU/mg (n=7) of mitochondrial protein in the acidotic rats and 92.49+/-6.79 mU/mg (n=7) in the control rats. Western blot analysis revealed a significant reduction (14%) in the level of SerDH protein content in the rat liver during acidosis. Immunohistochemical analysis showed that SerDH location did not change in response to chronic metabolic acidosis and confirmed previous results on SerDH protein levels. Moreover, the SerDH mRNA level, estimated by RT-PCR, was also significantly 33.8% lower than in control. These results suggest that during experimental acidosis a specific repression of rat-liver SerDH gene transcription could result, lowering the amount and activity of this enzyme. The changes found in SerDH expression are part of an overall metabolic response of liver to maintain acid-base homeostasis during acidosis.

Enzymatic and biochemical properties of a novel human serine dehydratase isoform

A cDNA clone similar to human serine dehydratase (SDH) is deposited in the GenBank/EMBL databases, but its structural and functional bases remain unknown. Despite the occurrence of mRNA, the expected protein level was found to be low in cultured cells. To learn about physicochemical properties of the protein, we expressed the cDNA in Escherichia coli, and compared the expressed protein with that of a hepatic SDH. The purified protein showed l-serine and l-threonine dehydratase activity, demonstrating to be an isoform of SDH. However, their Km and Vmax constants were different in a range of two-order. Removal of Pro128 from the hepatic SDH consisting of 328 residues, which is missing in the corresponding position of the isoform consisting of 329 residues, significantly changed the Michaelis constants and Kd value for pyridoxal 5'-phosphate, whereas addition of a proline residue to the isoform was without effect. These findings suggest the difference in the structures of the active sites of the two enzymes. Another striking feature was that the expressed level of the isoform in E. coli was 7-fold lower than that of the hepatic SDH. Substitution of Val for Leu287 in the isoform dramatically increased the protein level. The high yield of the mutated isoform was also confirmed by the in vitro transcription and translation experiment. The poor expression of the isoform could be explained by the more stable secondary structure of the mRNA than that of the hepatic SDH mRNA. The present findings may provide a clue as to why the protein level in cultured cells is low.