The Saturation Kinetics Model (SKM) is useful in describing many physiological responses as functions of a limiting dietary nutrient. However, as nutrients are fed at higher dietary concentrations, responses become inhibited and diminish from their usual plateaus. By adding an inhibition constant (Ks) to the SKM in a manner consistent with substrate inhibition (based on enzyme kinetics), it becomes possible to predict the inhibited portions of the nutrient-response curve. To test this, rats were fed diets of graded levels of casein (0-75%) or lysine (0-6.2%), and weight gains and food intakes were measured daily for up to 2 wk. The inhibition form of the SKM was able to predict the complete response range of each experiment, producing a Ks (weight gain) at a dietary level of 50.60% for casein and 7.56% for lysine. It was also possible to set up an upper and lower dietary nutrient concentration that encompassed the 100% response range for each response, thereby giving an inhibition or toxicity index of 2.02 for casein and 4.98 for lysine. This index allows one to set nutritional requirement levels precisely, optimizing responses without moving into inhibiting or toxic ranges of nutrients. Based on growth response curves, requirements were 25.61% for casein and 1.97% for lysine.

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Liver microsomal hydroxylation of 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea was shown to occur on the cyclohexyl ring at positions 3 and 4. Four metabolites were isolated by selective solvent extraction and purifed by high-pressure liquid chromatography. cis-4-, trans-4-, cis-3-, and trans-3-OH derivatives of 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea were synthesized and their chromatographic, mass spectral, and nuclear magnetic resonance characteristics matched those of the metabolites. The position of ring hydroxylation and the identity of each geometric isomer were established by nuclear magnetic resonance using a shift reagent in conjunction with spin decoupling techniques. Microsomes from rats pretreated with phenobarbital showed a sixfold increase in hydroxylation rate (19.5 vs. 3.3 nmol per mg per min). The induction was quite selective for cis-4 hydroxylation (19-fold); however, induction of trans-4 (threefold), cis-3 (threefold), and trans-3 (twofold) hydroxylation did occur. Quantitatively the cis-4-hydroxy metabolite was 67of the total product by phenobarbital-induced microsomes and 21% for normal microsomes. Microsomes from animals pretreated wit- 3-methyl-cholanthrene gave about the same rate and product distribution that normal microsomes gave. A mixture of 80% carbon monoxide-20% oxygen inhibited formation of all four hydroxy metabolites with the inhibition ranging from 55 to 78%.

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Events accompanying electron transport in the membrane fraction of liver and other tissues have led us to propose a specific function for α‐tocopherol based on a sequence of biochemical changes we observed to occur in these membranes and on pertinent information from other laboratories. The activity of a membrane‐bound enzyme system (TPNH oxidase) which involves transport of electrons from substrate to oxygen, has been shown to promote simultaneous formation of peroxide functions on the β position polyunsaturated fatty acids (PUFA) of phospholipids in the membrane. The phospholipid peroxides then undergo a chain cleavage reaction producing phospholipids containing a variety of carbonyl moieties in the β position. The process results in marked alteration of the membrane structure. During the overall reaction α‐tocopherol present in the membrane is converted to a compound more polar than tocopheryl quinone and the conversion is dependent on the same enzymic factors promoting the phospholipid alterations. The membrane alteration process is enhanced in microsomes from animals fed diets containing relatively high levels of PUFA or diets low in α‐tocopherol, and is diminished by low levels of dietary PUFA or relatively high levels of α‐tocopherol. The experimental data indicate that enzymic electron transport associated with TPNH oxidation by the microsomal membrane involves free radical functions. The latter apparently can promote extensive peroxidative alterations of phospholipids that result in structural changes in the membrane unless adequate α‐tocopherol is present in this organelle. This system appears to be part of the microsomal drug metabolizing system.