Abstract
Chitinolytic activities were measured in two fish species having different feeding habits, chub mackerel (Scomber japonicus) and silver croaker (Pennahia argentata). Chitinase (an endo-type chitinolytic enzyme) activity was measured using pNP-(GlcNAc)n (n = 2, 3) as substrates; its level was significantly high in the stomachs of both species, as well as in the gills, intestine, pyloric appendage, testis, and liver of chub mackerel and in the spleen, kidney, pyloric appendage, ovaries, heart, and liver of silver croaker. β-N-Acetylhexosaminidase (an exo-type chitinolytic enzyme) activity was measured using pNP-(GlcNAc) as a substrate; it was detected at high levels in many parts apart from the digestive tracts of both species. The optimum pH for chitinase activity was 3.0 - 5.0 in the stomachs of both species, 4.0 in the liver of chub mackerel, and 4.0 and 8.0 in the kidney of silver croaker. Full-length cDNAs encoding two chitinase isozymes were obtained from the stomachs of the two fish species: SjChi-1 (1604 bp) and SjChi-2 (1512 bp) from chub mackerel and PaChi-1 (1630 bp) and PaChi-2 (1606 bp) from silver croaker. Expression analysis of these genes in the organs of the two species revealed strong expression of SjChi-1 in the stomach of chub mackerel and that of PaChi-1 andPaChi-2 in the stomach of silver croaker. The difference in the expression pattern of these genes is likely attributed to the difference in the feeding habits of the two fish species. Our results suggested the presence of novel chitinases in the two species.
Highlights
Chitin is an amino polysaccharide containing N-acetyl-d-glucosamine (GlcNAc) units connected with β-1,4 linkages
Chitinolytic activity showed the highest levels of chitinase activity against pNP-(GlcNAc)2 and pNP-(GlcNAc)3 in the stomachs of chub mackerel and silver croaker
In chub mackerel, chitinase activity was observed in the pyloric appendage, which is significantly developed as a part of the digestive system, suggesting that chitinase contributes to the digestion of
Summary
Chitin is an amino polysaccharide containing N-acetyl-d-glucosamine (GlcNAc) units connected with β-1,4 linkages. Chitinolytic enzymes degrade chitin and are classified according to the degradation mechanisms as endo-type chitinolytic enzyme, which produce (GlcNAc)n by randomly degrading chitin internally, and exo-type chitinolytic enzyme, which produce GlcNAc by degrading chitin sequentially from the nonreducing ends [9]. The former is referred to as chitinase (EC 3.2.1.14) and belongs to the glycoside hydrolase GH family 18 or 19 based on the amino acid sequence homology within the catalytic domain [10]. The latter is referred to as β-N-acetylhexosaminidase (Hex) (EC 3.2.1.52) [11]
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