During the processing of heshiko, a fermented mackerel product, a rapid increase in peptide content in the extract and a remarkable decrease in the IC 50 (the inhibitor concentration to inhibit 50% of enzyme activity) as an index of the angiotensin I-converting enzyme (ACE) inhibitory activity were observed. Systolic blood pressure (SBP) in spontaneously hypertensive rats (SHR) decreased between 2 and 4 h after single oral administration of heshiko extract at a dose of 10 mg/ kg as a peptide, and SBP recovered its initial level by 8 h. For single doses of extract at three different levels (5, 10 and 50 mg/kg), SBP similarly decreased after between 2 and 4 h. The decreased SBP at 50 mg/kg was almost equal to that at 10 mg/kg, indicating a low dose dependency for heshiko extract. Through successive administration of heshiko extract or its desalted extract at 10 mg/kg for 10 days, SBP decreased 7 days after the start of administration and it recovered its initial level 5 days after stopping administration. During these periods, the change in ACE activity in blood plasma from SHR administered the extract roughly corresponded to that of SBP, suggesting that ACE inhibition was related to a decrease in SBP. For long-term administration of the extract to 5-week-old SHR for 70 days, SBP decreased 28 days after the start of administration. The decreased SBP remained low for 28 days after stopping administration, whereas the decreased ACE activity recovered its initial level. These results suggest that heshiko extract influences not only ACE inhibition, but also other systems that regulate blood pressure.KEY WORDS: angiotensin I-converting enzyme inhibition, antihypertensive effect, fermented mackerel, heshiko, peptide, spontaneously hypertensive rat.
During the processing of mackerel narezushi , the fish body was strongly dehydrated by permeation of salt, the low pH of fish meat, and pressure applied to the fish and rice mixture. In the proximate components, moisture, protein and lipid flowed out from the fish meat into the rice mixture, and sugar permeated from the rice mixture into the fish meat. That the total amount of outflow was larger than that of permeation into the fish is thought to contribute to the decrease in fish body weight during processing. In the extractive components, although the nucleotides completely decomposed, free amino acids and peptides increased remarkably because of the decomposition of proteins in fish meat during processing. Especially, six kinds of free amino acid, Glu, Asp, Gly, Ala, Leu, and Ile, which are thought to be the taste components in marine resources, increased. Organic acid increased remarkably because of the fermentation of rice and permeation into the fish meat. The marked increase of the extractive components is thought to contribute to the umami taste and the sour taste of narezushi .KEY WORDS: extractive component, fermented food, free amino acid, mackerel, narezushi , organic acid, peptide, proximate composition.
Permeability of sodium chloride (NaCl) into fish meat strips caused by soaking was compared with that of sorbitol as a function of osmotic pressure of the soaking solution. Increase in the osmotic pressure of the soaking solution similarly promoted the permeation of both compounds. However, the increasing rate of the permeation of sorbitol gradually decreased with rising osmotic pressure, while that of NaCl was constant under the osmotic pressure range tested. Higher dispersing ability of NaCl in meat strips was thought to promote further permeation of NaCl. In contrast, soaking in NaCl solution with a lower concentration markedly increased the moisture content of the meat strips. Further increase in the osmotic pressure of NaCl in the soaking solution caused the osmotic dehydration similar to sorbitol. Dewatering effect of NaCl was considerably lower than that of sorbitol at the same osmotic pressure. Combined use of NaCl and sorbitol in the soaking solution gave no further promoting action on the permeation of each compound, nor on their dewatering effects. These results suggested that the osmotic dehydration was competitive with the increase in the water‐holding capacity of myofibrillar proteins caused by the permeation of NaCl.
Heshiko extract was separated into peptide and amino acid fractions by ion-exchange column chromatography. Heshiko extract and these fractions were administered to rats in a diet enriched with lipid and cholesterol for 30 days. In the heshiko extract group, increase of triglyceride, total cholesterol, and low-density lipoprotein (LDL)-cholesterol levels in plasma and accumulation of total lipids in the liver were suppressed, while amounts of both lipid and cholesterol excreted to feces were increased. Administration of heshiko extract tended to increase fecal bile acids and promoted the activity of cholesterol 7a-hydroxylase, the rate-limiting enzyme in the synthesis of bile acid from cholesterol in the liver. However, activity of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, the rate-limiting enzyme in the cholesterol synthesis system in the liver, decreased due to regulation by the feedback of lipid transportation from diet to the liver. The same effect was observed in the peptide and free amino acid fraction groups. These data suggest that both excretion of cholesterol and bile acid to feces and promotion of cholesterol 7a-hydroxylase activity are related to the hypocholesterolemic effects of heshiko extract and that both peptides and amino acids contribute to that effect.
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