**2. Results**

#### *2.1. Characterization of UPE*

As shown in Table 1, the main component of UPE is carbohydrate, accounting for about 52.61% of the total composition; in addition, UPE contains uronic acid and sulfate group. By molecular weight analysis, the average molecular weight of UPE is about 891.25 kDa. According to the analysis of monosaccharide composition (Figure 1b,c), it is mainly composed of Rha, GlcA, glucose (Glc), and Xyl, and the content of Rha is the highest (45.33%). In addition, through FT-IR (Figure 1a), the signals at 1640, 1427, and 1054 cm−<sup>1</sup> are respectively the absorption peaks of the stretching vibration of carbonyl group C=O, the absorption peaks of the stretching vibration of carboxyl group C-O, and the absorption peaks of the O-H angular vibration. These three absorption peaks are characteristic absorption peaks of uronic acids [11]. The absorption peaks of 1259 and 843 cm−<sup>1</sup> are the characteristic absorption peaks of the sulfate group, where 1259 cm−<sup>1</sup> is the absorption peak of S=O stretching vibration and 843 cm-1 is the absorption peak of C-O-S stretching vibration. According to the above results, it was proved that both sulfuric acid and uronic acid were present in UPE, which were the main characteristics of ulvan.

**Table 1.** Molecular weight, composition, and monosaccharide composition of UPE.


**Figure 1.** Structural characterization of UPE. (**a**) FT-IR. (**b**) Standard monosaccharide. 1-Man, 2-Rha, 3-GlcA, 4-GalA, 5-Lac (internal standard), 6-Glc, 7-Gal, 8-Xyl, 9-Ara, 10-Fuc. (**c**) Monosaccharide composition of UPE. 2-Rha, 3-GlcA, 5-Lac (internal standard), 6-Glc, 8-Xyl.

#### *2.2. Effect of UPE on Organ Index*

Organ index is the ratio of a certain organ to its body weight. After an animal's body is damaged, organ weight will change, and organ coefficient will also change. Therefore, the viscera ratio can intuitively judge the severity of body damage. The body weight and kidney weight of mice are shown in Table 2. The renal index of mice is shown in Figure 2a. The renal index of mice injected with D-gal decreased significantly (*p* < 0.01) when the initial body weight was basically the same. However, after UPE treatment, the renal index was significantly increased (*p* < 0.01), which indicated that UPE alleviated the phenomenon of renal atrophy to a certain extent and had a protective effect on the kidney.

**Table 2.** The different groups for body weights and kidney weights.


\* *p* < 0.05, \*\* *p* < 0.01 compared with BC group; ## *p* < 0.01 compared with NC group. BC: blank control, NC: negative control, PC: positive control, LD: low-dose UPE (50 mg/kg), HD: high-dose UPE (300 mg/kg).

**Figure 2.** Effect of UPE on organ index and serum Scr, BUN, and Cys-C levels. (**a**) Organ index. (**b**) Scr content. (**c**) BUN content. (**d**) Cys-C content. The data are expressed as mean ± standard deviation (*n* = 9). \*\* *p* < 0.01 compared with BC group; ## *p* < 0.01 compared with NC group. BC: blank control, NC: negative control, PC: positive control, LD: low-dose UPE (50 mg/kg), HD: high-dose UPE (300 mg/kg).
