Molecular and functional explication of thioredoxin mitochondrial-like protein (Trx-2) from big-belly seahorse (Hippocampus abdominalis) and expression upon immune provocation

Thioredoxin (Trx) is a small ubiquitous multifunctional protein with a characteristic WCGPC thiol-disulfide active site that is conserved through evolution. Trx plays a crucial role in the antioxidant defense system. Further, it is involved in a variety of biological functions including gene expression, apoptosis, and growth regulation. Trx exists in several forms, with the cytosolic (Trx-1) and mitochondrial (Trx-2) forms being the most predominant. In this study, the mitochondrial Trx protein (HaTrx-2), from the big-belly seahorse (Hippocampus abdominalis) was characterized, and its molecular features and functional properties were investigated. The cDNA sequence of HaTrx-2 consists of a 519 bp ORF, and it encodes a polypeptide of 172 amino acids. This protein has a calculated molecular mass of 18.8 kDa and a calculated isoelectric point (pI) of 7.80. The highest values of identity (78.7%) and similarity (86.2%) were observed with Fundulus heteroclitus Trx-2 from the pairwise alignment results. The phylogenetic analysis revealed that HaTrx-2 is closely clustered with teleost fishes. The qPCR results showed that HaTrx-2 was prevalently expressed at various levels in all the tissues examined. The ovary showed the highest expression, followed by the brain and kidney. HaTrx-2 showed varying mRNA expression levels during the immune challenge experiment, depending on the type of tissue and the time interval. Our results confirmed the antioxidant property of HaTrx-2 by performing the MCO assay, DPPH radical scavenging activity, and cell viability assays. Further, an insulin disulfide reduction assay revealed the dithiol remove the enzymatic activity of HaTrx-2. Altogether these results indicate that HaTrx-2 plays indispensable roles in the regulation of oxidative stress and immune response in the seahorse.