**1. Introduction**

Constituting approximately 0.51% to 1.08% of the body weight, the kidneys are an energydemanding organ and receive approximately 20–25% of the cardiac output [1–3]. Every 24 min, it filters a volume equal to that of whole plasma volume; and every 6 h, it filters a volume equal to that of total body water [3]. Given this workload, the kidney needs a large amount of ATP produced by mitochondria, which, unfortunately, also generate reactive oxygen species (ROS) as metabolic byproducts [4–6]. Therefore, the kidney is under constant attack from ROS. Such is indeed the case in diabetic kidney disease (DKD) whereby oxidative stress is elevated and mitochondrial dysfunction is aggravated, leading to renal injury [7,8]. DKD, also known as diabetic nephropathy [9–13], is a common complication of diabetic mellitus, including both type 1 and type 2 diabetes. While type 1 diabetes is caused by lack of insulin due to pancreatic β cell destruction [14–16], type 2 diabetes could be caused by insulin resistance or insulin deficiency [17–22]. The hallmark of diabetes is a persistent high blood glucose content (hyperglycemia) that can damage a variety of tissues and cells [15,21–23]. In the kidney, renal microvascular structures are the major targets of high blood glucose [24–27]. Additionally, given the facts that the kidney is the organ where mature or active form of vitamin D is made [28–30] and erythropoiesis erythropoietin is produced [31–33], DKD can also lead to vitamin D deficiency and anemia [34–40]. Therefore, while there is great understanding of the pathophysiology and progression of DKD, novel and effective treatment approaches are still needed as current therapeutic options remain limited.

While many mechanisms underlie the pathogenesis of DKD including protein kinase C pathway [41,42], hexosamine pathway [43,44], formation of advanced glycation end products [45,46] and the polyol pathway [47,48]; at the molecular level, redox imbalance of NADH/NAD+ caused by deranged glucose metabolism [49–51] may stand out as a distinct mechanism of diabetic kidney injury [52–55]. This is because electrons from breakdown

**Citation:** Yan, L.-J. NADH/NAD<sup>+</sup> Redox Imbalance and Diabetic Kidney Disease. *Biomolecules* **2021**, *11*, 730. https://doi.org/10.3390/ biom11050730

Academic Editor: Theodoros Eleftheriadis

Received: 22 April 2021 Accepted: 12 May 2021 Published: 14 May 2021

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of glucose and other nutrients such as fatty acids and amino acids are stored in NADH using NAD<sup>+</sup> as the electron acceptor [56–58]. Therefore, a key feature of diabetes mellitus is oversupply of NADH and under supply of NAD+ [48,51,59].

#### **2. Sources of Elevated NADH in Diabetes**

In addition to the conventional metabolic pathways that extract electrons by breaking the chemical bonds in carbohydrates and fatty acids (Figure 1), other glucose utilization pathways are activated by hyperglycemia [47]. One of these pathways is the polyol pathway [60,61] (Figure 2), which can burn up to 30% of the glucose pool in a diabetic patient [62]. This pathway converts glucose to fructose and also converts NADPH to NADH. There is also an intermediate product known as sorbitol, which could accumulate and impair cellular osmosis in the kidney [63,64]. While the first reaction from glucose to sorbitol is catalyzed by aldose reductase, the second reaction from sorbitol to fructose is catalyzed by sorbitol dehydrogenase. In this pathway, aldose reductase is the ratelimiting enzyme that has a high Km value for glucose [65]. Therefore, numerous studies have focused on aldose reductase as a potential therapeutic target in diabetes [66–71]. In particular, attention has been paid to develop small molecule compounds that can inhibit aldose reductase [72–76] to prevent accumulation of sorbitol and fructose and to prevent build-up of NADH, the elevation of which can perturb NADH/NAD<sup>+</sup> redox balance, initiating reductive stress and oxidative stress. Furthermore, the contribution of the polyol pathway to diabetes development has been demonstrated by the use of aldose reductase animal models whereby lack of aldose reductase prevents the development of diabetes [76]. It should be noted that this NADH/NAD<sup>+</sup> redox imbalance is also termed as pseudohypoxia in diabetes [77,78] because hypoxia and ischemia often leads to NADH accumulation and NAD<sup>+</sup> depletion [79–81]. It should also be noted that endogenous production of fructose via the polyol pathway has been shown to cause increased fructose and fructose-1-phosphate contents in the kidney, leading to aggravation of DKD [82].

**Figure 1.** The conventional metabolic pathways that generate NADH from NAD+. Shown are the glycolytic pathway, fatty acid oxidation, and the Krebs cycle. These are the major pathways that store electrons in NADH by breaking the chemical bonds in dietary components including glucose, fatty acids. Enzymes involved in direct production of NADH are also indicated in the diagram.

**Figure 2.** The polyol pathway. This pathway contains two reactions. The first reaction converting glucose to sorbitol is catalyzed by aldose reductase. This enzyme is rate-limiting for the whole pathway. The second reaction converting sorbitol to fructose is catalyzed by sorbitol dehydrogenase. The final products are NADH and fructose, and sorbitol is an intermediate product. Note that NADPH is consumed by aldose reductase in the first reaction. Additionally, accumulation of sorbitol in the kidney could cause osmotic problems for nephrons [63,64].
