Different physiological functions of free D- and L-alanine in three body parts of the intertidal sipunculid Phascolosoma arcuatum |
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Authors: | W P Low W T Ong Y K Ip |
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Institution: | (1) Department of Zoology, National University of Singapore, 10 Kent Ridge Crescent, Kent Ridge, 0511 Singapore, Republic of Singapore |
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Abstract: | Free D- and L-alanine contents were comparable in the body wall and introvert cum retractor muscles of Phascolosoma arcuatum. In contrast, the content of free D-alanine in the internal organs was twice that of free L-alanine. Since alanine aminotrans-ferase from P. arcuatum was L-alanine specific, D-alanine appeared to be synthesized from L-alanine through the action of alanine racemase. Alanine racemase activity was higher in the D-alanine-forming direction in the three body parts of P. arcuatum. In addition, the ratio of D L/L D racemase activity in the internal organs was the lowest among the body parts studied. These results indicate that free D-alanine might be of lesser importance than the free D-isomer to the internal organs as compared to the body wall and introvert cum retractor muscles. Indeed, L-alanine inhibited pyruvate kinase from the body wall and introvert cum retractor muscles but had no effect on the pyruvate kinase from the internal organs. Furthermore, the activity of alanopine dehydrogenase present in the internal organs was significantly lower than those of the body wall and introvert cum retractor muscles. L-Alanine was an essential substrate for alanopine formation in the body wall and introvert cum retractor muscles during hypoxia since alanopine dehydrogenases from these body parts were L-alanine specific. When P. arcuatum was confronted with hypo-osmotic stress, the free D-alanine/total free alanine ratio in the internal organs increased approximately from 0.6 to 0.8 as the total free alanine content decreased. In comparison, those ratios in the body wall and introvert cum retractor muscles remained relatively constant. It was concluded that D- and D-alanine had different physiological functions in the three body parts of P. arcuatum.Abbreviations
ADP
adenosine-5 -diphosphate
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ADH
alanopine dehydrogenase
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ALT
alanine aminotransferase
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AOD
amino acid oxidase
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BW
body wall
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EDTA
ethylenediaminetetra-acetic acid
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EGT A
ethylene glyco-bis ( -aminoethyl ether)
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N,N,N,N
tetra-acetic acid
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ICRM
introvert cum retractor muscles
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IO
internal organs
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I
50
inhibitor concentration producing 50% inhibition of enzyme activity
- -KG
-ketoglutarate
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LDH
lactate dehydrogenase
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NAD
nicotinamide adenine dinucleotide
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NADH
nicotinamide adenine dinucleotide (reduced form)
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PEP
phosphoenolpy-ruvate
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PEPCK
phosphoenolpyruvate carboxykinase
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PK
pyruvate kinase
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PMSF
phenylmethylsulphonyl fluoride
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SE
standard error
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SW
sea water
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TCA
trichloroacetic acid |
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Keywords: | Alanine Amino acid Racemase Sipunculid Phascolosoma arcuatum |
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