## 1 Introduction  
Under normal conditions, reactive oxygen species (ROS) are effectively eliminated by antioxidant defense systems, such as antioxidant enzymes and nonenzymatic factors. However, the balance between the generation and elimination of ROS is broken under pathological conditions, and uncontrolled generation of free radicals can attack proteins, membrane lipids, and DNA, which induce many health disorders including cancer, neurodegenerative, coronary heart diseases, and Alzheimer's disease [1; 2]. In addition, free radical-mediated lipid oxidation can react with proteins, amino acids, vitamins, and cholesterol during food processing, transportation and storage. The reaction will result in loss of color, nutrition, and functionality as well as undesirable off-flavors and toxic compounds [3]. There is increasing demand for antioxidants in the pharmaceutical and health food industries as well as the food processing and preservation industries. Therefore, some artificial antioxidants including butylated hydroxytoluene (BHT), butylated hydroxyanisole (BHA), and tertiary butylhydroquinone (TBHQ) show stronger antioxidant activities and have been widely applied in food preservation for retarding lipid oxidation [4; 5]. However, these synthetic antioxidants might cause liver damage and carcinogenesis [5]. Therefore, there has been a major interest in searching for new, natural, and efficient antioxidants from various sources as alternatives to synthetic antioxidants.  
Bioactive peptides consist of 2 to 20 amino acid residues and are inactive in the amino acid sequence of their parent proteins, and they can be released by in vitro enzymatic hydrolysis without destroying their nutritional value [3]. These low molecular weight (MW) peptides are considered to have easy absorption, high activity, and contain no hazardous immunoreactions [5; 6]. Recently, seafood-derived peptides with antioxidant properties have been prepared and identified from different aquatic organisms, such as the dark muscle of tuna [7; 8], skin of Alaskan pollock [9], skin and head of bluefin leatherjacket [10; 11], viscera and carcass of Nile tilapia [12], gonad of jellyfish [13], monkfish muscle [14], skin of Nile tilapia [15], and pectoral fin of salmon [16]. This research indicates that seafood-derived protein hydrolysates and peptides have strong antioxidant activity and could serve as functional ingredients in food systems to protect food quality by reducing oxidative stress. In addition, bioactive protein hydrolysates and/or peptides can be applied as ingredients of functional foods due to their low cost, safety, and high nutritional and physiological value [3].  
Spotless smoothhound (Mustelus griseus) belongs to Chondrichthyes Carcharhiniformes Triakidae Mustelus, mainly distributed in the Pacific Northwest. In our previous research, five antioxidant peptides including GAA, GFVG, GIISHR, ELLI, and KFPE were isolated from ethanol-soluble proteins hydrolysate of spotless smoothhound muscle, and they showed strong 2,2-diphenyl-1-picrylhydrazyl radicals (DPPH\(\bullet\)), hydroxyl radicals (HO\(\bullet\)), 2,2'-azino-bis-3-ethylbenzothazoline-6-sulfonic acid radicals (ABTS\({}^{\bullet}\bullet\)), and superoxide anion radicals (O\({}_{2}^{-}\bullet\)) scavenging activities [17]. Acid-soluble collagen and its hydrolysate were prepared from scalloped hammerhead cartilages, and their physicochemical properties were also characterized [18; 19]. However, there was no research focusing on the antioxidant peptides from spotless smoothhound cartilages. Therefore, three antioxidant peptides were prepared from trypsin-hydrolysate of spotless smoothhound cartilages, and their radical scavenging activities and inhibiting abilities on lipid peroxidation, protection on H\({}_{2}\)O\({}_{2}\)-induced HepG2 cells from oxidative stress were evaluated in this study.