2002 年 30 巻 4 号 p. 221-228
Several free D-amino acids, which were thought not to exist in eukaryotes, have recently been detected in various aquatic invertebrates. In particular, free D-alanine was found in large amount (3-50 mmol/g wet wt.) in the tissues of several crustaceans and bivalve mollusks. Under high salinity stress, these animals largely accumulate D- and L-alanine irrespective of species, together with several other non-essential L-amino acids of which increases are species dependent. In these species, D- alanine is proven to be one of the major compatible osmolytes responsible for the intracellular isosmotic regulation or cell volume regulation. D-Alanine is also accumulated with L-alanine and inorganic ions in the tissues of Japanese mitten crab Eriocheir japonicus during gonadal maturation in the river and during spawning downstream migration toward the sea.
Under hypoxia stress, red swamp crayfish Procambarus clarkii in creased D- and L-alanine in muscle and hepatopancreas in addition to the in crease of lactate. The in crease is much higher in seawater than in freshwater. Thus, D- and L-alanine are possible to be anaerobic end products during prolonged hypoxia of this species and other invertebrates.
Alanine racemase [EC 5.1.1.1] has been proven to catalyze the interconversion of D- and L-alanine in crustaceans and bivalve mollusks and purified from two crustacean species and a mollusk. The enzyme isolated from the muscle of black tiger prawn Penaeus monodon is a dimer having molecular mass of 90 kDa. Several partial amino acid sequences of peptide fragments obtained from the isolated enzyme showed positive homologies from 52 to 76% (identity from 31 to 45%) with bacterial counterparts and a catalytic tyrosine residue of the bacterial enzyme was also retained in the prawn one, indicating that alanine racemase gene is well conserved from bacteria to invertebrates.
