|B. Sc.||1980||North-Eastern Hill University, Shillong|
|M. Sc.||1982||North-Eastern Hill University, Shillong|
|Ph. D.||1987||North-Eastern Hill University, Shillong|
Specialization: Biochemical Adaptation and Metabolic Regulation
|1.||Professor||NEHU, Shillong||13.05.2003||Till date||14 years & 4 months|
|2.||Reader||NEHU, Shillong||18.12.1996||12.05.2003||6 years & 5 months|
|3.||Senior Lecturer||NEHU, Shillong||07.04.1993||16.12.1996||3 years & 8 months|
|4.||Lecturer||NEHU, Shillong||06.05.1988||06.04.1993||5 years|
Ph.D.: 18 (eighteen)
M.Phil.: 1 (one)
13. Major findings in research:
Prof. Saha has shown for the first time the unique presence of a functional ornithine-urea cycle (OUC) in two freshwater air-breathing catfish that are predominantly available in Indian subcontinent, such as Heteropneustes fossilis, Clarias batrachus (Saha and Ratha, 1987,1989; Saha et al., 1999), which otherwise is known to be non-functional or absent in typical fresh water teleosts. This unique finding by Prof. Saha has contradicted the earlier “gene deletion” hypothesis proposed by Brown and Cohen (1960) based on their findings of non-occurrence of functional OUC in fresh water teleosts, may be because of deletion of some genes required for OUC during their adaptive evolution in fresh water. In contrast, Prof. Saha and his group showed the existence of a functional OUC in air-breathing catfish, regulation of which depends on variable environmental constraints that they face regularly in natural habitats. For example, while living inside the mud-peat for months during the drought season under water restricted condition (Ratha et al. 1995; Saha et al., 2001), high concentration of ambient ammonia in stagnant water bodies due to evaporative water loss in summer (Saha and Ratha, 1990,1994; Saha and Das, 1999; Saha et al., 1995, 2003, 2007), and in alkaline condition (Saha et al.,2002), which compel them to switch over from ammoniotelic to ureotelic mode of nitrogen excretion by inducing the OUC enzyme activities. Although such unique adaptation was witnessed in amphibians, but not reported in any teleost species. In addition to this, Prof. Saha revealed existence of a functional OUC in non-hepatic tissues other than its usual presence in hepatic tissues, such as in kidney and muscle of these catfish as a special adaptation (Saha and Ratha, 1987, 1989; Saha et al., 1999), which is not found in higher vertebrates. Prof. Saha has also shown that these catfish have the capacity to accumulate very high concentration of ammonia while this is not so in mammals, where excess level of plasma ammonia may lead to coma and death. Further, these catfish have the ability to convert the accumulated ammonia to glutamine and various non-essential amino acids very efficiently (Saha et al., 2000, 2002, 2007), as another unique adaptational strategy existing in these catfish. Another recent important contribution by Prof. Saha and his group is in relation to the occurrence of three glutamine synthetase (GS) genes expressed differentially in different tissues in catfish in contrast mammalian system, where the GS enzyme is encoded by only one gene, and all the three GS genes get induced specifically in different tissues under ammonia stress in high ambient ammonia (Banerjee et al. 2018). His group has reported mitochondrial localization of both the isoforms of arginase (ARG 1 and 2), the last enzyme of the OUC, in C. batrachus liver, thereby suggesting that urea is exclusively synthetized inside the mitochondria of hepatic cells of catfish (Banerjee et al., 2017). This is again a unique observation in contrast to mammals, where urea is synthesised primarily in the cytosol of hepatic cells due to cytosolic localization of ARG, suggesting a regulatory role of mitochondrial urea synthesis in catfish.
Prof. Saha made a significant revelation regarding the presence of two types of carbamyol phosphate synthetase (CPS), the CPS I (ammonia- and N-acetyl-L-glutamate-dependent), which is normally present in higher vertebrates such as in mammals and amphibians, in addition to the presence of a typical fish-type CPS III (glutamine- and N-acetyl-L-glutamate dependent) activity in air-breathing catfish (Saha et al., 1997, 1999, 2007). This is again another novel contribution of showing the presence of both the OUC-related CPSes (CPS I and III) in a single vertebrate species, which was not observed together in any other vertebrate. It was suggested that glutamine- and N-acetyl glutamate-dependent CPS III, found in lower vertebrates, is the evolutionary precursor to ammonia- and N-acetyl-L glutamate-dependent CPS I of ureotelic mammalian and amphibian species (Mommsen and Walsh, 1989). Thus, the presence of both types of OUC-related CPS activities in these two catfish probably has evolutionary significance with relation to the evolution of OUC in vertebrates, in addition to their physiological significance of efficient conversion of toxic ammonia to urea via the OUC by involving both the isoforms of CPS under ammonia stress.
Prof. N. Saha and his group have also demonstrated the induction of nitric oxide (NO) synthesis by inducing the inducible nitric oxide synthase (NOS) gene under ammonia stress during exposure to high ambient ammonia and during mud-dwelling in air-breathing catfish (H. fossilis and C. batrachus) (Choudhury and Saha, 2012a,b; Kumati et al. 2019). He further suggested that the enhanced production of NO under ammonia stress, which is known to play diverse physiological functions in mammals under environmental stresses, is also playing a significant role in catfish in defending against the various ammonia-induced stressors as another adaptational strategy. His group has also recently demonstrated that enhanced production of endogenous NO can have antioxidant activity against the nanoparticle-induced oxidative stress in hepatocytes of air-breathing catfish (Koner et al. 2019).
Administrative Experience/Posts & responsibilities held:
List of publications: Total: 75 (Total IF: 190; Total citations: 2040; H index: 25)
National 16; International: 69