Current assessment practices for noncancer end points.

The need for assessing noncancer risks for agents to which humans are routinely exposed indoors arises from the large amount of time spent indoors (i.e., employed persons spend about 60% of their time at home indoors, 30% at work indoors, and 5% in transit). Sources of air pollutants include heating and cooling systems, combustion appliances, personal use products, furnishings, tobacco products, pesticides, bioeffluents from humans and animals, and other microbial contamination such as toxins from molds. The purpose of this paper is to describe current dose-response assessment methods applicable to assessing risk following exposure to indoor air pollutants. The role of structure-activity relationships in hazard identification is also described. Risk assessments from exposure to indoor air pollutants require exposure assessments and dose-response assessments. Dose-response assessment methodologies include the inhalation reference concentration (RfC), structure-activity relationships, dose-response models, and the decision analytic approach. The RfC is an estimate (with uncertainty spanning perhaps an order of magnitude) of a daily exposure to the human population (including sensitive subgroups) that is likely to be without an appreciable risk of deleterious effects during a lifetime. The current RfC method provides guidelines for making the necessary dosimetric adjustments for gases and aerosols. Human equivalent concentrations for no-observed-adverse-effect levels in animals are determined by using mathematical relationships that adjust for regional deposition, solubility, ventilation rate, and blood:air partition coefficients. The RfC methodology exists as an interim methodology. Future scientific advancements are expected to further refine the approach.

Pharmacokinetic modeling of the lung burden from repeated inhalation of nickel aerosols.

The saturable nature of the clearance of soluble nickel compounds from the lung was studied by repeated exposures of rats to respirable submicron-size nickel aerosols. Using Michaelis-Menten type kinetics for removal of nickel lung burdens and a constant rate of deposition, the lung nickel burdens were simulated by computer. The computer simulation was used to design a repeated exposure regimen to test further the hypothesis of saturable clearance. Male Sprague-Dawley rats were exposed for 2 h/day to nickel chloride aerosols at either 90 or 400 micrograms Ni/m3 for up to 14 days. During the 22 h between exposures and up to 3 days post-exposure rats were kept in clean air. The particle size of the aerosol ranged from 0.7 to 0.9 micron mass median aerodynamic diameter with a geometric standard deviation of 1.2-1.4. A steady-state nickel lung burden was observed at 90 micrograms/m3, as predicted from computer modeling, while lung burdens continued to increase with repeated exposure to 400 micrograms Ni/m3. The best fit for the experimental data was obtained with a maximum clearance velocity (Vmax) of 34.6 ng Ni/g X h and a Michaelis-Menten constant for transport (Kt) of 1380 ng Ni/g. The percentage of submicron nickel chloride aerosols retained in the lung was 6.9%. These data support the hypothesis of a saturable clearance mechanism for soluble nickel and provide physiological constants useful for estimating human health risks from nickel inhalation.

Effects of adenine and its isomer 4-aminopyrazolo-[3,4-d]-pyrimidine on 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced mortality in rats.

Adult male Fischer rats were given a single po dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) equal to 2 times the LD50 to increase the serum and liver lipid concentrations and to induce mortality. In addition, animals were given 4-aminopyrazolo-[3,4-d]-pyrimidine (4APP), an agent that decreases serum lipids, or adenine (Ad), an agent that prevents the formation of fatty liver, to examine the relationship between changes in lipids and TCDD-induced mortality. The principal effect of 4APP on TCDD-induced mortality (325 micrograms TCDD/kg body wt) was that it shortened the mean time to death. In contrast, Ad stimulated feed consumption and decreased body weight loss, but the mean times to death were similar for TCDD and TCDD + Ad animals. Based on these mortality studies, 4APP, but not Ad, affects the TCDD-induced mortality in Fischer rats. The TCDD-induced sensitivity to 4APP, based on decreased mean time to death, implies that blocking the release and/or synthesis of triglyceride-rich lipoproteins by the liver, and the subsequent decrease in serum lipids, may play an important role in the TCDD-induced mortality. The increase in serum triglyceride associated with TCDD exposure appears to be essential in providing metabolic energy under circumstances where lipoprotein retrieval is reduced.

Uptake of the components of phenylalanylphenylalanine and maltose by intestinal epithelium.

The observed rate of phenylalanine absorption into rat intestinal rings with 0.5 or 5.0 mM phenylalanine is greater than that for absorption of phenylalanine from 0.25 or 2.5 mM Phe-Phe, respectively. With the amino acid phenylalanine, V for absorption is the same whether Na+ is present (149 mM) or absent, but the concentration at which the half-maximal transport rate occurred (Kt) is greater in the absence of Na+. For Phe-Phe, the V decreases in the absence of Na+ whilst Kt is not influenced by the Na+ concentration. The different effect of Na+ on Phe and Phe-Phe transport indicates that the absorptive mechanism for Phe-Phe is different from that for phenylalanine. Absorption of a mixture of [U-14C]Phe-[he and Phe-[G-3H]Phe showed identical rates of uptake of the carboxyl and amino terminal amino acids. Studies of transport of radioactive maltose showed that the rates of uptake of the reducing and non-reducing glucosyl moieties are identical. Radioactive maltose absorption is not inhibited by glucose oxidase. These results provide evidence that in intestinal epithelium, hydrolysis of Phe-Phe and maltose does not occur on the cell surface with release of the hydrolyzed products to the medium. Rather, hydrolysis and release of the reaction products occur at a point on the cytosol side of a diffusion barrier located in the brush border membrane.

Isolation and characterization of four peptide hydrolases from the brush border of rat intestinal mucosa.

Peptide hydrolases (EC 3.4.-.-) were solubilized from purified brush borders of rat intestinal mucosa by papain digestion. Three peptide hydrolases, I, II, and III, with different substrate specificities were isolated by means of DEAE-cellulose chromatography and preparative acrylamide gel electrophoresis. On repeat preparative acrylamide gel electrophoresis under slightly different conditions, enzyme II was resolved into two proteins, IIa and IIb, with vary similar, possibly identical, substrate specificities. Efforts to discover additional brush border peptide hydrolases revealed none. Studies using more than 50 substrates showed that enzyme I was most active against Met-Met, Met-Ala, and Met-Phe while enzyme II was most active against Phe-Gly, Phe-Ser, and Leu-Gly-Gly, and enzyme III most rapidly hydrolyzed Gly-Leu, Leu-Gly, and Met-Gly. Efforts to discover substrates which are highly discriminating for each enzyme were partly successful. Thus, a number of substrates including leucine amide, leucyl-beta-naphthylamide and Phe-Asp were hydrolyzed almost exclusively (95% or more) by enzyme II while Gly-Leu was similarly specific for enzyme III. No substrate highly discriminating for enzyme I was discovered. Ion-exchange chromatography resulted in increases in specific activity of 10- and 120-fold for enzymes II and III, respectively. By sequential use of ion-exchange chromatography and preparative acrylamide gel electrophoresis, each of the three enzymes was partially purified to the point that they were free of contaminating disaccharidases and enzymes I and II gave single dense bands on analytical acrylamide gel electrophoresis while enzyme III gave a single dense band plus one additional faint protein band. Under appropriate conditions, analytical gel electrophoresis also resolved enzyme II into two bands with enzyme activity. The three enzymes were isolated from intestinal brush borders of germ-free rats indicating that none of the enzymes is of bacterial origin. With Phe-Gly as substrate, pH optima for enzymes I, II, and III were 8.0, 8.0, and 8.5, respectively. Molecular weights determined by gel filtration were 283 000, 284 000, and 134 000, respectively. Studies of activation by metal ions and inhibition by metal ion chelators suggested that the activity of each of the enzymes is dependent on a relatively tightly bound metal cofactor. Peptide hydrolases of the intestinal mucosa play an essential role in protein digestion. The studies presented here help to clarify the total number and substrate specificities of these enzymes in the rat brush border.