**1. Introduction**

Collagen is an important structural protein of connective tissue, and it is also a principal component of the natural extracellular matrix (ECM) that plays a dominant role in providing overall tissue stiffness and integrity [1]. The main feature of collagen is its triple helical structure. In collagen type I, this structure consists of two identical polypeptide chains, α1, and one polypeptide chain, α2, with each chain containing one or more repeating amino-acid motifs (Gly–X–Y), where X is proline or hydroxyproline and Y represents any amino acid [2,3]. So far, 29 types of collagen (I–XXIX) have been identified and characterized. Among them, fibril-forming type I collagen with a high structural order and high stiffness is the most widely distributed type of collagen in connective tissue, accounting for 80–85% of collagen in the body [4,5]. Due to its excellent biocompatibility, low antigenicity, and high biodegradability, type I collagen is regarded as one of the promising biomaterials and is widely used in tissue engineering and the pharmaceutical and biomedical industry [6,7].

Collagen's preferred sources are the skin and tendons of bovine and porcine. However, as collagen of mammals has the risk of triggering an immune reaction and transferring zoonosis and transmissible spongiform encephalopathies, marine collagen has attracted interest in recent years [2,8]. Marine collagen has lower gelling and melting temperatures than mammalian collagen, but marine collagen is cheaper to extract and easier to prepare than mammalian collagen [8,9]. Marine collagen, such as that from sponges, jellyfish, squids, octopuses, cuttlefish, and fish skin, bone, and scales, comes from both marine vertebrates and invertebrates [9,10]. There is grea<sup>t</sup> demand for marine collagen, and this is now the main source of collagen globally. Sourour Addad et al. (2011) obtained

**Citation:** Chen, J.; Wang, G.; Li, Y. Preparation and Characterization of Thermally Stable Collagens from the Scales of Lizardfish (*Synodus macrops*).*Mar. Drugs* **2021**, *19*, 597. https:// doi.org/10.3390/md19110597

Academic Editor: Sik Yoon

Received: 25 September 2021 Accepted: 19 October 2021 Published: 21 October 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

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collagen from Jellyfish [11], Tziveleka et al. (2017) isolated the collagen from marine sponges *Axinella cannabina* and *Suberites carnosus* [12] and skin and Cruz-López et al. (2018) extracted collagen from gulf corvina skin and swim bladder [10]. As reported in the literature, the marine collagen market is expected to reach USD 983.84 million by 2025, with a compound annual growth rate of 7.4% [13]. Marine collagen, compared with collagen from terrestrial sources, is more easily extracted [2], has high biocompatibility [5], is without the risks of animal diseases and pathogens; has a higher absorption capacity (up to 1.5 times more efficient entry into the body), and is not associated with religious and ethical restrictions [14,15]. This provides an opportunity for fish scales. Namely, fish scales are the waste product from the fish processing industry, and they represent on average 2% of fish body weight [16]. The poor biodegradability of scales makes them difficult to be managed as waste [16]. However, scales are a safe and good source of marine type I collagen [17]. Therefore, the extraction of type I collagen from scales may be beneficial in terms of both economic and environmental benefits, and it could possibly drive the development of new industries. Type I collagen from scales has gained increasing interest, and scales are widely regarded as a promising source of collagen [17]. Many successful extractions of collagen from scales have been reported, including tilapia scales collagen [18], gourami scales collagen [19], and miiuy croakers scales collagen [20]. Lizardfish (*Synodus macrops*) is a common economic fish species in China, and there have been no studies about lizardfish scales collagen.

Therefore, in this study, we isolated collagen from lizardfish scales by using acid and enzymatic extraction methods; characterized the physicochemical properties, structural properties, and thermal stability of acid-soluble collagen (ASC) and pepsin-soluble collagen (PSC), and investigated the rheological properties, and cell viability, all of which might provide useful information for the development and application of marine collagen.
