Hirofumi Arai

Phosphatidylcholine levels and their fatty acid compositions in teleost tissues and squid muscle

Abstract The lipid content and composition of phosphatidylcholine (PC) and fatty acids were determined in tissues (muscle, liver, heart and gonads) of 27 species of teleosts. We also measured PC and fatty acids in muscle of 6 species of squid. Contents of phospholipid and PC in the muscle tissue ranged between 0.54 and 0.94%, 0.30 and 0.72%, respectively. Muscular PC was dominant in surf smelt, filefish, lizard fish, white croaker, counting for 75–82% of muscular phospholipid, Pacific salmon, Pacific pomfret, pollack and plaice had 24–32% phosphatidylethanolamine which was considerably higher than the other sample fishes. Species specificity of n −6 and n −3 polyunsaturated fatty acid (PUFA) distribution in the muscular PC was noted: coastal and reef dwelling fish species contained higher proportions of n −6 PUFA, while a higher content of n −3 PUFA was characteristic for the migratory teleosts.

Fish Protein Decreases Serum Cholesterol in Rats by Inhibition of Cholesterol and Bile Acid Absorption

Fish protein has been shown to decrease serum cholesterol content by inhibiting absorption of cholesterol and bile acid in laboratory animals, though the mechanism underlying this effect is not yet fully understood. The purpose of this study was to elucidate the mechanism underlying the inhibition of cholesterol and bile acid absorption following fish protein intake. Male Wistar rats were divided into 2 dietary groups of 7 rats each, 1 group receiving a diet consisting of 20% casein and the other receiving a diet consisting of 10% casein and 10% fish protein. Both experimental diets also contained 0.5% cholesterol and 0.1% sodium cholate. After the rats had been on their respective diets for 4 wk, their serum and liver cholesterol contents and fecal cholesterol, bile acid, and nitrogen excretion contents were measured. Fish protein consumption decreased serum and liver cholesterol content and increased fecal cholesterol and bile acid excretion and simultaneously increased fecal nitrogen excretion. In addition, fish protein hydrolyzate prepared by in vitro digestion had lower micellar solubility of cholesterol and higher binding capacity for bile acids compared with casein hydrolyzate. These results suggest that the hypocholesterolemic effect of fish protein is mediated by increased fecal cholesterol and bile acid excretion, which is due to the digestion products of fish protein having reduced micellar solubility of cholesterol and increased bile acid binding capacity.

Effects of Dietary Fish Protein on Serum and Liver Lipid Concentrations in Rats and the Expression of Hepatic Genes Involved in Lipid Metabolism

Dietary proteins influence the lipid metabolism of human subjects and animals. This study evaluated the effects of fish protein on lipid metabolism in rats. Alaska pollock fillets, widely supplied as raw materials of surimi, were used as fish protein. As parameters of lipid metabolism, cholesterol and triacylglycerol concentrations in the serum and liver, the fecal excretion of bile acids, and the hepatic expression of genes encoding proteins involved in lipid homeostasis were examined. Rats fed fish protein showed decreased cholesterol concentrations in the serum and liver, and fecal bile acid and cholesterol concentrations were increased. This was caused by the increased expression of cholesterol 7alpha-hydroxylase (CYP7A1) as the digested fish protein inhibited the absorption of bile acid and cholesterol in the small intestine. In addition, it was found that dietary fish protein affects the farnesoid X receptor/small heterodimer partner-dependent pathway, which is negatively regulated by the decreased reabsorption of bile acid. Furthermore, it increased the binding to the promoter of CYP7A1 through activated liver receptor homologue-1.

Oxidative Modification of Lipoproteins

Lipoproteins consist of lipids and apolipoproteins that have functional roles in lipid metabolism. It has been suggested that oxidation of lipoproteins by reactive oxygen species (ROS) may be involved in the inception of various diseases. In particular, the relationship between low-density lipoprotein (LDL) oxidation and atherosclerosis has been studied in great detail. The main target molecules of lipoprotein oxidation are polyunsaturated fatty acid residues of lipids and apolipoproteins. Extensive investigations have characterized oxidative modifications of apolipoprotein B100 (apo B100) in LDL. Furthermore, modifications of apo B100 by oxidized lipids have been confirmed in oxidized LDL and atherosclerotic lesions using immunological techniques. In this chapter, characteristics and oxidation mechanisms of lipoproteins by ROS are described from in vitro and in vivo studies. Oxidative modifications of apo B100 by lipid hydroperoxides, major products of lipid peroxidation at the early stage, are principally reported.

Fish Protein Hydrolysates Affect Cholesterol Metabolism in Rats Fed Non-Cholesterol and High-Cholesterol Diets

Fish consumption is well known to provide health benefits in both experimental animals and human subjects. Numerous studies have demonstrated the beneficial effects of various protein hydrolysates on lipid metabolism. In this context, this study examined the effect of fish protein hydrolysates (FPH) on cholesterol metabolism compared with the effect of casein. FPHs were prepared from Alaska pollock meat using papain as a protease. Male Wistar rats were divided into the following four dietary groups of seven rats each: either casein (20%) or FPH (10%) + casein (10%), with or without 0.5% cholesterol and 0.1% sodium cholate. Serum and liver lipid levels, fecal cholesterol and bile acid excretions, and the hepatic expression of genes encoding proteins involved in cholesterol homeostasis were examined. In rats fed the FPH diets compared with casein diets with or without cholesterol and sodium cholate, the indexes of cholesterol metabolism-namely, serum cholesterol, triglyceride, and low-density lipoprotein-cholesterol levels-were significantly lower, whereas fecal cholesterol and bile acid excretions were higher. Rats fed the FPH diets compared with casein with cholesterol exhibited a lower liver cholesterol level via an increased liver cholesterol 7α-hydroxylase (CYP7A1) expression level. This study demonstrates that the intake of FPH has hypocholesterolemic effects through the enhancement of fecal cholesterol and bile acid excretions and CYP7A1 expression levels. Therefore, fish peptides prepared by papain digestion might provide health benefits by decreasing the cholesterol content in the blood, which would contribute to the prevention of circulatory system diseases such as arteriosclerosis.

Effect of dietary protamine on lipid metabolism in rats

Protamine has been widely used as a pharmaceutical product and natural food preservative. However, few studies have been conducted to assess the beneficial function of dietary protamine. This study examined the effects of dietary salmon protamine on serum and liver lipid levels and the expression levels of genes encoding proteins involved in lipid homeostasis in the liver of rats. Groups of male Wistar rats were fed AIN93G diet containing 2% or 5% protamine. After 4 weeks of feeding these diets, markedly decreased serum and liver cholesterol (CHOL) and triacylglycerol levels were noted. Increased activity of liver carnitine palmitoyltransferase-2 and acyl-CoA oxidase, which are key enzymes of fatty acid β-oxidation in the mitochondria and peroxisomes, was found in rats fed on protamine. Furthermore, rats fed protamine showed enhanced fecal excretion of CHOL and bile acid and increased liver mRNA expression levels of ATP-binding cassette (ABC) G5 and ABCG8, which form heterodimers and play a major role in the secretion of CHOL into bile. The decrease in triacylglycerol levels in protamine-fed rats was due to the enhancement of liver β-oxidation. Furthermore, rats fed protamine exhibited decreased CHOL levels through the suppression of CHOL and bile acid absorption and the enhancement of CHOL secretion into bile. These results suggest that dietary protamine has beneficial effects that may aid in the prevention of lifestyle-related diseases such as hyperlipidemia and atherosclerosis.

Response of osteoblast‐like MC3T3‐E1 cells on bioactive titanium fabricated by a chemical treatment process using a calcium‐phosphate slurry

We recently developed a chemical treatment process using a calcium-phosphate slurry for fabricating new layers consisting of hydroxyapatite and titanium dioxide (TiO2) on titanium (Ti) substrate. In this study, the response of osteoblast-like MC3T3-E1 cells on Ti substrate treated with a calcium-phosphate slurry was investigated to elucidate its behavior in a biological environment. The cellular adhesiveness and proliferation capacity did not differ significantly between the treated and untreated Ti substrates, suggesting that the slurry treatment did not cause cytotoxicity. The slurry treatment did not affect the increase in alkaline phosphatase activity after the induction of cell differentiation, whereas it was found to be significantly advantageous for the calcification behavior on the slurry-treated Ti substrate. In consequence, the hard-tissue compatibility of Ti is expected to be improved by the chemical treatment process using a calcium-phosphate slurry.

Salmon Protamine Decreases Serum and Liver Lipid Contents by Inhibiting Lipid Absorption in an In Vitro Gastrointestinal Digestion Model and in Rats

Protamine has been used as an antiheparin drug and a natural preservative in various food products. However, limited studies have evaluated the physicochemical and functional properties of protamine. Hence, we assessed the mechanisms underlying the inhibition of lipid absorption following salmon protamine intake in in vitro and in vivo studies. In initial experiments, a salmon protamine hydrolyzate (PH) was prepared using in vitro simulated gastrointestinal digestion suppressed pancreatic lipase activity and micellar cholesterol solubility. This PH had higher bile acid-binding capacity and emulsion breakdown activity than casein hydrolyzate and l-arginine. However, the hypolipidemic functions of protamine were dramatically reduced by pancreatin digestion. In further experiments, groups of male Wistar rats were fed an AIN-93G diet containing 5% (wt/wt) salmon protamine or a protamine amino acid mixture. After 4 wk of feeding with experimental diets, reductions in serum and liver triacylglycerol (TAG) and cholesterol contents were observed in the presence of protamine, reflecting inhibition of TAG, cholesterol, and bile acid absorption. These data suggest that the formation of insoluble PH-bile acid complexes is critical before the bile acid-binding capacity is reduced. Therefore, dietary salmon protamine may ameliorate lifestyle-related diseases such as hyperlipidemia and obesity.

Combination effect of phospholipids and n-3 polyunsaturated fatty acids on rat cholesterol metabolism

This study evaluated phospholipids (PLs) containing n-3 polyunsaturated fatty acids (n-3 PUFAs) for their specific inherent effects and effects due to a combination of the presence of glycerophosphate structure and n-3 PUFAs on cholesterol metabolism in rats. Rats were fed a diet of AIN-93G containing soybean oil (SO, 7%), SO (5.8%)+fish oil (1.2%), SO (5.2%)+soybean PLs (1.8%), SO (5.2%)+PLs containing n-3 PUFAs (1.8%), and SO (4.0%)+fish oil (1.2%)+soybean PLs (1.8%). Diets with PLs containing n-3 PUFAs, and soybean PLs in combination with fish oil, resulted in decreased serum and liver cholesterol levels through enhancement of fecal cholesterol excretion and suppression of liver sterol regulatory element binding protein-2 mRNA expression compared with the diet containing soybean oil alone. This study shows that soybean PLs with added triacylglycerol that included n-3 PUFAs have the same effects on cholesterol metabolism as PLs containing n-3 PUFAs, and that these could be of benefit to people.

Effect of Simultaneous Intake of Fish Protein and Fish Oil on Cholesterol Metabolism in Rats Fed High-Cholesterol Diets

This study demonstrated the effect of the simultaneous dietary administration of fish protein and fish oil, two macronutrients found in fish meat, on cholesterol metabolism in rats. Male Wistar rats were divided into four groups and fed an AIN-93G modified hypercholesterolmic diet with casein (20%) + soybean oil (7%), casein (10%) + fish protein (10%) + soybean oil (7%), casein (20%) + soybean oil (5%) + fish oil (2%), and casein (10%) + fish protein (10%) + soybean oil (5%) + fish oil (2%) for four weeks. Cholesterol metabolism was measured through serum and liver cholesterol contents, fecal cholesterol and bile acid excretion levels, and liver mRNA expression levels of enzymes and nuclear receptors involved in cholesterol homeostasis. Dietary fish protein decreased serum and liver cholesterol contents, perhaps through increasing fecal cholesterol and bile acid excretion and liver cholesterol 7 -hydroxylase expression level. Dietary fish oil, on the other hand, decreased liver cholesterol content, perhaps due to the suppression of cholesterol synthesis through a decrease in the 3-hydroxy-3methylglutaryl-coenzyme A reductase expression level; the serum cholesterol content was unchanged. This study found that the simultaneous dietary administration of fish protein and fish oil, which is achieved by the intake of intact fish muscle, has hypocholesterolemic effects that help prevent hyper- lipidemia and atherosclerosis.