Altered expression and glucocorticoid-inducibility of hepatic CYP3A and CYP2B enzymes in male rats fed diets containing soy protein isolate.

Hepatic CYP3A and CYP2B enzymes were studied in male Sprague-Dawley rats derived from 5-7 litters fed diets in which the protein source was either casein or soy protein isolate. At age 65 d, rats were gavaged with corn oil (vehicle) or 50 mg/kg dexamethasone. Hepatic expression of CYP3A and CYP2B1 mRNA, apoprotein and associated monooxygenase activities were measured. Consumption of soy diets significantly increased monooxygenase activity toward the following: the CYP3A substrates erythromycin and ethylmorphine N-demethylase; corticosterone and testosterone 6beta-hydroxylase; and apoprotein and mRNA expression of CYP3A2 (P < 0.05). Dexamethasone significantly induced turnover of erythromycin and testosterone, expression of CYP3A apoprotein, and expression of CYP3A1 and CYP3A2 mRNA (P < 0.05). In addition, significant diet-inducer interactions were observed in the expression of CYP3A apoprotein and activities toward ethylmorphine, corticosterone and testosterone (P < 0.05). Significant diet-inducer interactions were also observed on CYP2B1-dependent pentoxyresorufin O-depentylase activity (P < 0.05). However, although dexamethasone significantly induced CYP2B1 expression at the apoprotein and mRNA level (P < 0.05), no significant diet effects were observed. These data suggest potential effects of soy consumption on the metabolism of a wide variety of CYP3A and CYP2B1 substrates, especially in situations involving coexposure to CYP inducers.     

Effects of feeding rats low protein diets containing casein or soy protein isolate supplemented with methionine or oligo-L-methionine

The effects of supplementing 8% casein or 10% soy protein isolate (SPI) diets with graded levels of oligo-L-methionine (a mixture of hexa- and heptapeptides, OM) or L-methionine (Met) were studied in rats to determine the reason for the difference in nutritional quality between proteins and corresponding amino acid mixtures. As the OM concentration of the casein-based diet was increased from 0.02% to 0.6%, maximum weight gain was attained at 0.2%, and the growth-promoting activity of OM was comparable to Met at all the corresponding levels tested. Liver fat began to accumulate when supplemental Met reached a level of 0.08% of the casein diet, but OM addition did not produce a fatty liver at dietary levels of less than 0.3%. When SPI was used as the dietary protein source, the effect of supplemental OM was significantly less than that of Met. Digestibility of OM (assessed by incremental portal plasma Met concentration) was measured 30 min after feeding the casein or SPI diet supplemented with 3% OM using rats fasted for 24 h. Plasma Met concentration was greatly increased in rats fed the casein plus OM diet compared with that of rats fed the SPI + OM diet. Similarly, the 30-min portal Met concentration significantly increased in response to the casein + OM diet compared with the SPI + OM diet regardless of the prefed proteins (25% casein and 25% SPI for 2 wk).(ABSTRACT TRUNCATED AT 250 WORDS)     

Absorption of calcium, magnesium, phosphorus, iron and zinc in growing male rats fed diets containing either phytate-free soybean protein or soybean protein isolate or casein

The effect of dietary phytate-free soybean protein (PFS) on intestinal mineral absorption and retention was examined in growing male rats using a three-day mineral balance technique. The rats were fed diets containing PFS, soybean protein isolate (SPI) or casein at a 20% level for 5 wk. Total calcium (Ca), magnesium (Mg), phosphorus (P), iron (Fe) and zinc (Zn) contents in diets were adjusted to 0.35, 0.05, 0.7, 0.0035 and 0.003%, respectively, by supplementation of the diet with their salts. Mineral absorption and retention ratios in rats fed the PFS diet were significantly higher than those in rats fed either the SPI or casein diet. These results suggest that PFS may be a promising dietary protein source for improving the mineral bioavailability in humans.     

Substitution of casein by beta-casein or of whey protein isolate by alpha-lactalbumin does not affect mineral balance in growing rats

Bovine milk protein fractions that enable modification of the protein composition and amino acid profile of infant formulas to mimic those of human milk have recently become available. To determine the effects on protein quality and mineral bioavailability of replacing casein by beta-casein and of whey protein isolate by alpha-lactalbumin, 4 groups of growing rats were fed for 3 wk diets containing 10% protein as 1) casein (control); 2) beta-casein; 3) casein:whey (40:60); or 4) beta-casein:alpha-lactalbumin (40:60). Protein quality, determined as protein efficiency ratio (PER), net protein utilization (NPU), biological value (BV) and protein digestibility (PD), as well as body weight gain, were higher (P < 0.05) with consumption of the whey-adapted diets [casein:whey (40:60); beta-casein:alpha-lactalbumin (40:60)] compared with the casein diets (casein; beta-casein); however, there were no differences between the 2 casein diets or between the 2 whey-adapted diets. Apparent absorption of minerals (Ca, P, Fe, Zn) from the whey-adapted diets was higher than that from the casein diets (P < 0.05); but again, no differences were observed when casein or whey protein isolate were replaced by beta-casein or alpha-lactalbumin, respectively. Thus, substitution of casein by beta-casein or of whey protein isolate by alpha-lactalbumin does not affect protein quality or mineral bioavailability as determined in growing rats.     

Feeding meals containing soy or whey protein after exercise stimulates protein synthesis and translation initiation in the skeletal muscle of male rats

The purpose of this investigation was to compare the early response of skeletal muscle protein synthesis and translation initiation following the ingestion of different protein sources after endurance exercise. Treadmill-acclimated rats were designated as either nonexercised controls (NEX) or treadmill exercised for 2 h at 26 m/min (approximately 75% VO2max) and then fed either carbohydrate only (EC), carbohydrate plus soy protein (ES), or carbohydrate plus whey protein (EW). One hour after exercise, serum insulin concentrations in EC, ES, and EW were greater than in NEX (P<0.05); the concentration in EW was greater than in EC, with that in ES intermediate. Serum concentrations of branched-chain amino acids in ES and EW were higher than in EC, but serum leucine and isoleucine in EW were higher than in ES (P<0.05). Nevertheless, both ES and EW promoted the fractional rate of skeletal muscle protein synthesis significantly more than EC. Likewise, compared with EC, both ES and EW increased formation of the mRNA cap binding complex eIF4F and stimulated phosphorylation of the translational repressor, 4E-BP1, the 70kD ribosomal protein S6 kinase (S6K1), and the mammalian target of rapamycin (mTOR) kinase at serine 2448. On the other hand, phosphorylation of S6K1 and mTOR was greater in EW than in ES (P<0.05). In conclusion, general protein synthesis and the mRNA cap binding step are promoted comparably by soy protein and whey protein in the skeletal muscle of exercised rats. Furthermore, the data suggest that mTOR signaling in skeletal muscle is acutely responsive to physiological variations in dietary amino acids.     

Plasma very low density lipoproteins from male rats fed casein or soybean protein diets: a comparison of fatty acid composition and influence on prostanoid production

In studies with male rats fed for 4 wk semipurified diets containing olive oil and casein or soybean protein, protein-dependent effects were observed in the fatty acid composition of the VLDL lipids, especially with regard to the pattern of polyunsaturated fatty acids. Compared with VLDL from rats fed soybean protein diet (S-VLDL), VLDL from casein-fed rats (C-VLDL) contained a greater level of oleic acid, and a reduced level of linoleic acid and arachidonic acid, in the phosphatidylcholine fraction. The proportion of 5,8,11-eicosatrienoic acid, 20:3 (n-9), varied among the different lipid classes. The highest concentration of this fatty acid (13% by weight of total fatty acids) was observed in the phosphatidylinositol fraction of C-VLDL. The level of linoleic acid was approximately halved in the triacylglycerol and cholesteryl ester fractions of C-VLDL compared with S-VLDL. When mouse peritoneal macrophages were incubated with different concentrations of S-VLDL, a saturable accumulation of 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha) and thromboxane B2 (TXB2) was observed in the cell medium. In contrast, very low levels of 6-keto-PGF1 alpha and TXB2 were observed in the cell medium of macrophages incubated with C-VLDL at different lipoprotein concentrations, suggesting that the composition of VLDL may play an important role in relation to cellular prostanoid metabolism.     

Induction of hepatic CYP1A in male F344/NCr rats by dietary exposure to Aroclor 1254: examination of immunochemical, RNA, catalytic, and pharmacokinetic endpoints.

Male F344/NCr rats were exposed to low dietary concentrations of Aroclor 1254 (0-33 ppm) for 7 days, following which the induction of selected hepatic drug metabolizing enzymes was monitored. CYP1A1, measured indirectly by assaying the O-dealkylation of ethoxyresorufin in 9000 g supernatants, was increased 1.5-, 3-, 8-, and 37-fold following 7 days of exposure to 1.0, 3.3, 10, and 33 ppm Aroclor, respectively. In contrast, the O-dealkylation of benzyloxyresorufin, an indirect measure of CYP2B1 activity, was increased approximately 4-fold following exposure to 33 ppm dietary Aroclor. Measurement of the non-P450-mediated activities epoxide hydrolase, DT-diaphorase, and aldehyde dehydrogenase (NADP+, benzaldehyde) revealed < 4-fold inductions following feeding of 33 ppm Aroclor. In view of the relatively high sensitivity of the CYP1A-specific catalytic endpoint as a biomarker for Aroclor exposure, alternative endpoints for detecting induction of this subfamily of P450 were also examined. The extent of in vivo CYP1A induction was assessed by measuring serum concentrations of zoxazolamine 150 min following an intraperitoneal dose of 100 mg/kg body wt. Slight decreases in serum zoxazolamine concentration were observed in rats exposed to as little as 1.0 ppm dietary Aroclor 1254, while profound decreases were seen in rats exposed to > or = to 10 ppm Aroclor. Immunodetection of CYP1A1 protein, with a monoclonal antibody directed against this cytochrome, revealed a 2.9-fold increase in rats exposed to as little as 1.0 ppm Aroclor, and approximately 10- and 44-fold increases following exposure to 3.3 and 10 ppm dietary Aroclor, respectively. Increases in total hepatocellular RNA coding for CYP1A1 and CYP1A2, quantified by hybridization to specific oligonucleotide probes, corresponded well to the increases in hepatic O-dealkylase activity for ethoxyresorufin (CYP1A1) and methoxyresorufin (CYP1A2), respectively. Thus, CYP1A induction, directly or indirectly measured with a variety of endpoints, represents a highly sensitive biomarker for exposure to relatively low doses of Aroclor 1254 in the rat.