Light-driven chloride ion transport by halorhodopsin from Natronobacterium pharaonis. 1. The photochemical cycle

The photochemical cycle of the light-driven chloride pump, halorhodopsin from N. pharaonis, is described by transient optical multichannel and single-wavelength spectroscopy in the visible, and in the infrared. Titration of a blue-shift of the absorption maximum upon addition of chloride describes a binding site with a KD of 1 mM. The reaction sequence after the all-trans to 13-cis photoisomerization of the retinal in this chloride binding form is itself dependent on chloride. At 2 M chloride it is described by the scheme: HR-->K<==>L<==>N-->HR that relaxes in a few milliseconds, and is very similar to the photocycle of bacteriorhodopsin under conditions where the retinal Schiff base cannot deprotonate. At lower chloride concentrations, e.g., 0.1 M, however, a red-shifted state termed O appears between N and HR, in equilibrium with N. The absorption spectra of K, L, N, and O are very similar to their counterparts in the bacteriorhodopsin photocycle. As in their equivalents in bacteriorhodopsin, in the N state the retinal is still 13-cis, but it is reisomerized in the O state to all-trans.

Functional significance of a protein conformation change at the cytoplasmic end of helix F during the bacteriorhodopsin photocycle

The second half of the photocycle of the light-driven proton pump bacteriorhodopsin includes proton transfers between D96 and the retinal Schiff base (the M to N reaction) and between the cytoplasmic surface and D96 (decay of the N intermediate). The inhibitory effects of decreased water activity and increased hydrostatic pressure have suggested that a conformational change resulting in greater hydration of the cytoplasmic region is required for proton transfer from D96 to the Schiff base, and have raised the possibility that the reversal of this process might be required for the subsequent reprotonation of D96 from the cytoplasmic surface. Tilt of the cytoplasmic end of helix F has been suggested by electron diffraction of the M intermediate. Introduction of bulky groups, such as various maleimide labels, to engineered cysteines at the cytoplasmic ends of helices A, B, C, E, and G produce only minor perturbation of the decays of M and N, but major changes in these reactions when the label is linked to helix F. In these samples the reprotonation of the Schiff base is accelerated and the reprotonation of D96 is strongly retarded. Cross-linking with benzophenone introduced at this location, but not at the others, causes the opposite change: the reprotonation of the Schiff base is greatly slowed while the reprotonation of D96 is accelerated. We conclude that, consistent with the structure from diffraction, the proton transfers in the second half of the photocycle are facilitated by motion of the cytoplasmic end of helix F, first away from the center of the protein and then back.

The complex extracellular domain regulates the deprotonation and reprotonation of the retinal Schiff base during the bacteriorhodopsin photocycle

During the L-->M reaction of the bacteriorhodopsin photocycle the proton of the retinal Schiff base is transferred to the anionic D85. This step, together with the subsequent reprotonation of the Schiff base from D96 in the M-->N reaction, results in the translocation of a proton across the membrane. The first of these critical proton transfers occurs in an extended hydrogen-bonded complex containing two negatively charged residues (D85 and D212), two positively charged groups (the Schiff base and R82), and coordinated water. We simplified this region by replacing D212 and R82 with neutral residues, leaving only the proton donor and acceptor as charged groups. The D212N/R82Q mutant shows essentially normal proton transport, but in the photocycle neither of this protein nor of the D212N/R82Q/D96N triple mutant does a deprotonated Schiff base (the M intermediate) accumulate. Instead, the photocycle contains only the K, L, and N intermediates. Infrared difference spectra of D212N/R82Q and D212N/R82Q/D96N demonstrate that although D96 becomes deprotonated in N, D85 remains unprotonated. On the other hand, M is produced at pH > 8, where according to independent evidence the L<==>M equilibrium should shift toward M. Likewise, M is restored in the photocycle when the retinal is replaced with the 14-fluoro analogue that lowers the pKa of the protonated Schiff base, and now D85 becomes protonated as in the wild type.(ABSTRACT TRUNCATED AT 250 WORDS)

HIV infection and AIDS risk behaviors among injecting drug users entering methadone treatment: an update

Trends in HIV infection and AIDS risk behaviors among injecting drug users (IDUs) were assessed through a series of nonblinded point-prevalence surveys conducted between 1987 and 1991 with admissions to methadone treatment in eight areas, including New York City; Asbury Park and Trenton, New Jersey; Philadelphia; Baltimore; Chicago; San Antonio, Texas; and Los Angeles County. Over the 5-year period, significant changes in HIV seropositivity were found in two of the eight cities, with seroprevalence decreasing in Asbury Park from 43.1 to 21.2% and increasing from 10.1 to 17.6% in Chicago. Initially high levels of injection-related risk behaviors decreased substantially across cohorts in most cities, except for San Antonio and Los Angeles, where risk levels remained high. Sexual risk behaviors continued at high levels in all cities, suggesting relatively little sexual risk reduction during the course of the study.

Conversion of bacteriorhodopsin into a chloride ion pump

In the light-driven proton pump bacteriorhodopsin, proton transfer from the retinal Schiff base to aspartate-85 is the crucial reaction of the transport cycle. In halorhodopsin, a light-driven chloride ion pump, the equivalent of residue 85 is threonine. When aspartate-85 was replaced with threonine, the mutated bacteriorhodopsin became a chloride ion pump when expressed in Halobacterium salinarium and, like halorhodopsin, actively transported chloride ions in the direction opposite from the proton pump. Chloride was bound to it, as revealed by large shifts of the absorption maximum of the chromophore, and its photointermediates included a red-shifted state in the millisecond time domain, with its amplitude and decay rate dependent on chloride concentration. Bacteriorhodopsin and halorhodopsin thus share a common transport mechanism, and the interaction of residue 85 with the retinal Schiff base determines the ionic specificity.

Post-traumatic stress disorder among inner city methadone maintenance patients

There has been little attention to the problem of violent stressors leading to post-traumatic stress disorder (PTSD) among opioid-dependent patients. In a sample of methadone maintenance patients, the prevalence of lifetime PTSD was determined to be 20% for women and 11% for men. The most common stressors reported were rape for women and seeing someone hurt or killed for men. Further research with methadone patients focused on PTSD should also ascertain current rates of PTSD and determine whether special treatments for PTSD and related problems of depression and suicidal ideation are necessary.

Changes in locus-of-control attitudes about drug misuse in a self-help group in a methadone maintenance clinic

Clients of a methadone-maintenance clinic in Brooklyn, New York participating in a clinically-guided self-help (CGSH) program plus standard treatment (methadone maintenance plus individual counseling) demonstrated statistically significant changes in locus-of-control beliefs, from external to internal causation, about personal responsibility for drug misuse. Members of two control groups--one participating in a didactic lecture program plus standard treatment and the other receiving only standard treatment--failed to demonstrate similar changes. This increase in internal locus of control in the CGSH group suggests the potential efficacy of CGSH as a relapse-prevention therapeutic technique.

Relationship of proton release at the extracellular surface to deprotonation of the schiff base in the bacteriorhodopsin photocycle

The surface potential of purple membranes and the release of protons during the bacteriorhodopsin photocycle have been studied with the covalently linked pH indicator dye, fluorescein. The titration of acidic lipids appears to cause the surface potential to be pH-dependent and causes other deviations from ideal behavior. If these anomalies are neglected, the appearance of protons can be followed by measuring the absorption change of fluorescein bound to various residues at the extracellular surface. Contrary to widely held assumption, the activation enthalpies of kinetic components, deuterium isotope effects in the time constants, and the consequences of the D85E, F208R, and D212N mutations demonstrate a lack of direct correlation between proton transfer from the buried retinal Schiff base to D85 and proton release at the surface. Depending on conditions and residue replacements, the proton release can occur at any time between the protonation of D85 and the recovery of the initial state. We conclude that once D85 is protonated the proton release at the extracellular protein surface is essentially independent of the chromophore reactions that follow. This finding is consistent with the recently suggested version of the alternating access mechanism of bacteriorhodopsin, in which the change of the accessibility of the Schiff base is to and away from D85 rather than to and away from the extracellular membrane surface.

Two-stage tuberculin skin testing in individuals with human immunodeficiency virus infection. Community Programs for Clinical Research on AIDS

In this study we estimated occurrence of the booster effect in a population infected with the human immunodeficiency virus (HIV) and assessed the relation between the booster effect, T-lymphocyte CD4 cell counts, tuberculosis risk categories, and HIV exposure categories. Patients were recruited from 13 participating sites of the Terry Beirn Community Programs for Clinical Research on AIDS (CPCRA). A two-stage tuberculin skin test was applied to 709 HIV-infected patients using the Mantoux method. An induration reading < 5 mm on the first test and > or = 5 on the second skin test defined the booster effect. Overall, 18 patients, or 2.7% (95% confidence interval, 1.6 to 4.2) experienced the booster effect. Boosted responses were seen in eight (2.1%) anergic patients, six (4.5%) nonanergic patients, and four (2.5%) with anergy status unknown. Boosting was noted in 1 of the 131 women enrolled. Age, race, CD4 cell count, injection drug use, anergy status, tuberculosis risk categories, and HIV exposure categories were not predictive of boosting. The booster effect occurs in a small percentage of HIV-infected patients tested, thus identifying small numbers of patients with latent tuberculosis infection. The two-stage procedure is probably of limited value in the diagnosis of latent tuberculosis in HIV-infected persons.

Bleeding injuries in professional football: estimating the risk for HIV transmission

OBJECTIVE

To determine the risk for bleeding injuries in professional football and to estimate the risk for transmission of the human immunodeficiency virus (HIV) through such injuries.

DESIGN

A prospective, observational study.

PARTICIPANTS

Professional football players from 11 teams of the National Football League were observed during 155 regular season games from September through December 1992.

MEASUREMENTS

The frequencies of bleeding injuries were calculated in association with environmental and athletic factors. Using this information, HIV prevalence, and data on transmission of HIV in other circumstances, the risk for transmission of HIV during football games was estimated.

RESULTS

575 bleeding injuries (average, 3.7 per game for each team) involving 538 players (average, 3.5 players on each team per game) were observed. Approximately 88% of the bleeding injuries were abrasions; the remainder were lacerations. Bleeding injuries were markedly more frequent during games played on artificial surfaces, during games played in domed stadiums, and on teams with a final win/loss percentage of 0.500 or lower. Using data on the prevalence of HIV among college men and rates of HIV transmission in the health care setting, the risk for HIV transmission to each player was estimated to be less than 1 per 85 million game contacts.

CONCLUSIONS

Although injuries occur in professional football competitions, bleeding injuries, especially lacerations, occur infrequently. We estimate that the risk for HIV transmission during such competition is extremely remote. The role of artificial playing surfaces on the incidence or severity of bleeding injuries should be investigated.

Interaction of tryptophan-182 with the retinal 9-methyl group in the L intermediate of bacteriorhodopsin

An intense indole N-H stretching vibrational band at 3486 cm-1 in the difference Fourier transform infrared spectrum is one of the characteristic features of the L intermediate of bacteriorhodopsin [Maeda, Sasaki, Ohkita, Simpson, & Herzfeld (1992) Biochemistry 31, 12543]. This band is now assigned to tryptophan-182. The Trp182-->Phe (W182F) protein shows specific features in the difference spectrum in the visible region upon L formation, and exhibits great delay in the L-M conversion. Fourier transform infrared difference spectra further indicate that while the intensity of the C-methyl in-plane bending vibration at 1009 cm-1 is lost in the L intermediate of the wild type, its intensity remains high in the W182F protein. The intensity of the N-H stretching vibration upon L formation is diminished considerably in an artificial bacteriorhodopsin containing 9-desmethylretinal. It also exhibits delayed M formation. These results suggest that Trp182 interacts with the retinal side chain through the 9-methyl group, and thereby affects the L-to-M conversion.

Survival and disease progression according to gender of patients with HIV infection. The Terry Beirn Community Programs for Clinical Research on AIDS

OBJECTIVE

To compare disease progression and mortality between women and men infected with human immunodeficiency virus (HIV).

DESIGN

Multicenter cohort.

SETTING

Seventeen community-based centers participating in the Terry Beirn Community Programs for Clinical Research on AIDS (CPCRA).

PATIENTS

A total of 768 women and 3779 men enrolled in one or more of 11 protocols between September 7, 1990, and September 30, 1993.

MAIN OUTCOME MEASURES

Survival and opportunistic events.

RESULTS

The median CD4+ cell count at enrollment into the cohort was 0.240 x 10(9)/L (240/microL) for women and 0.137 x 10(9)/L for men (P < .001). Compared with men, women were younger (36 vs 38 years), more likely to be African American or Hispanic (78% vs 44%), and more likely to have reported a history of injection drug use (49% vs 27%). Women had been followed up for a median of 14.5 months and men for 15.5 months. The adjusted relative risk (RR) for death among women compared with men was 1.33 (95% confidence interval [CI], 1.06 to 1.67; P = .01) and for disease progression (including death) was 0.97 (95% CI, 0.82 to 1.15; P = .72). Women were at increased risk for bacterial pneumonia (RR, 1.38; 95% CI, 1.05 to 1.92) and at reduced risk for the development of Kaposi's sarcoma (RR, 0.16; 95% CI, 0.04 to 0.65) and oral hairy leukoplakia (RR, 0.54; 95% CI, 0.31 to 0.94). The increased risk of death and bacterial pneumonia for women compared with men was primarily evident among those with a history of injection drug use (RR, 1.68 for death, 95% CI, 1.20 to 2.35, P = .003; RR, 1.53 for bacterial pneumonia, 95% CI, 1.03 to 2.29, P = .04). Among patients without a history of disease progression at entry, death was the first event reported for more women than men (27.5% vs 12.2%).

CONCLUSIONS

Compared with men, HIV-infected women in the CPCRA were at increased risk of death but not disease progression. Risks of most incident opportunistic diseases were similar for women and men; however, women were at an increased risk of bacterial pneumonia. These findings may reflect differential access to health care and standard treatments or different socioeconomic status and social support for women compared with men.

Energy coupling in an ion pump. The reprotonation switch of bacteriorhodopsin

The active site of an ion pump must communicate alternately with the two opposite membrane surfaces. In the light-driven proton pump, bacteriorhodopsin, the retinal Schiff base is first the proton donor to D85 (with access to the extracellular side), and then it becomes the acceptor of the proton of D96 (with access to the cytoplasmic side). This "reprotonation switch" has been associated with a protein conformation change observed during the photocycle. When D85 is replaced with asparagine, the pKa value of the Schiff base is lowered from above 13 to about 9. We determined the direction of the loss or gain of the Schiff base proton in unphotolyzed and in photoexcited D85N, and the D85N/D96N and D85N/D96A double mutants, in order to understand the intrinsic and the induced connectivities of the Schiff base to the two membrane surfaces. The influence of D96 mutations on proton exchange and on acceleration of proton shuttling to the surface by azide indicated that in either case the access of the Schiff base on D85N mutants is to the cytoplasmic side. In the wild-type protein (but with the pKa of the Schiff base lowered by 13-trifluoromethyl retinal substitution) the results suggested that the Schiff base can communicate also with the extracellular side. Raising the pH without illumination of D85N so as to deprotonate the Schiff base caused the same, or nearly the same, change of X-ray scattering as observed when the Schiff base deprotonates during the wild-type photocycle. The results link the charge state of the active site to the global protein conformation and to the connectivity of the Schiff base proton to the membrane surfaces. Their relationship suggests that the conformation of the unphotolyzed wild-type protein is stabilized by coulombic interaction of the Schiff base with its counter-ion. A proton is translocated across the membrane after light-induced transfer of the Schiff base proton to D85, because the protein assumes an alternative conformation that separates the donor from the acceptor and opens new conduction pathways between the active site and the two membrane surfaces.

The retinal Schiff base-counterion complex of bacteriorhodopsin: changed geometry during the photocycle is a cause of proton transfer to aspartate 85

Bacteriorhodopsin contains all-trans-retinal linked via a protonated Schiff base to K216. The proton transport in this pump is initiated by all-trans to 13-cis photoisomerization of the retinal and the ensuing transfer of the Schiff base proton to D85. Changed geometrical relationship of the Schiff base and D85 after the photoisomerization is a possible reason for the proton transfer. We introduced small volume/shape changes with site-specific mutagenesis of residues V49 and A53 that contact the side chain of K216, in order to force the Schiff base into somewhat different positions relative to D85. Earlier [Zimányi, L., Váró, G., Chang, M., Ni, B., Needleman, R., & Lanyi, J. K. (1992) Biochemistry 31, 8535-8543] we had described the kinetics of absorbance changes in the microsecond to millisecond time range after photoexcitation with the scheme L<-->M1<-->M2 + H+ (where the first equilibrium is the internal proton transfer and the second is proton release on the extracellular surface). Testing it at various pH values with mutants, where selected rate constants are changed, now confirms the validity of this scheme. The kinetics of the M state thus allowed examination of the transient equilibrium that develops in the L<-->M1 reaction and represents the redistribution of the proton between the Schiff base and D85. From the structure of the protein, the V49A and V49M residue replacements were both predicted to cause decreased alignment of the Schiff base and D85, and indeed we found that they both changed the equilibrium toward the protonated Schiff base. In contrast, the residue replacements A53V and A53G were predicted to move the Schiff base in opposite directions, away from and closer to alignment with D85, respectively. The former indeed changed the equilibrium toward the protonated Schiff base and the latter toward the deprotonated Schiff base. In addition, the hydroxyl stretch band of a bound water in the L state was affected by all mutations that disfavor proton transfer to D85. We conclude that the geometry of the proton donor and acceptor in the Schiff base-D85 pair, mediated by bound water, is a determinant of the proton transfer equilibrium.

The proton transfers in the cytoplasmic domain of bacteriorhodopsin are facilitated by a cluster of interacting residues

The stepwise internal proton transfer reactions across the membrane, and the release and uptake at the surface, are the elementary steps that together constitute the transport mechanism in a proton pump. Although the proton donor and acceptor residues can be usually identified, the directionality and the energetics of the proton transfer must be determined to a large extent also by interactions of these with neighboring groups. We have examined the roles of residues D96, T46 and R227 in proton transfers during the photocycle of bacteriorhodopsin near its cytoplasmic surface, and in general the relationship between the reprotonation of the Schiff base and the subsequent proton uptake from the cytoplasmic side. The phenotypes of single and double mutants suggest close functional interaction among D96, T46, R227, and probably internal bound water. Measurements of the free energies of activation indicate that mechanistic interpretation of the rates changed by residue replacements is hindered by a general tendency toward lowered activation enthalpies in the mutated proteins. There is less ambiguity in the free energy levels of the photointermediates. It appears from these that the inhibitory and stimulatory influences of T46 and R227, respectively, on D96 as a proton donor compensate one another and ensure the effective reprotonation of the Schiff base. T46 and D96 mediate, in turn, proton uptake at the cytoplasmic surface. Although ultimately this will reprotonate D96, the observation of proton uptake from the bulk in R82Q without reprotonation of the aspartate residue suggests that the direct proton acceptor is not D96. The results thus indicate that the passage of the proton from the surface to the Schiff base is facilitated by multiple residue and water interactions in the cytoplasmic domain.

HIV/AIDS policies and sports: the National Football League

With the assistance of highly respected experts in the field of HIV/AIDS, the NFL has developed a comprehensive policy that should decrease the spread of HIV and any other blood-borne pathogens among its players and medical staff. Transmission of HIV infection is likely to be rare in the NFL. This is supported by the fact that in over 10 yr of the AIDS epidemic, the CDC has not attributed one AIDS case to athletic competition. Whatever the rate of HIV infection, on-the-field transmission is certainly less frequent than hepatitis B, which is manyfold more transmissible than HIV. Based on these facts, a player with HIV infection poses virtually no threat to others or himself by further athletic participation in the NFL. In addition, the relatively short average playing career of an NFL player and the extended period between HIV contraction and development of AIDS symptoms decrease the prospect that a player's HIV-positive status would affect his athletic performance. Consequently, HIV testing should remain voluntary, and continued participation in the NFL of HIV-infected players should remain a private decision between the player and his physician.

Interviewer severity ratings and composite scores of the ASI: a further look

The psychometric characteristics of the Addiction Severity Index (ASI) interviewer severity ratings (ISRs) and composite scores (CSs) were examined for a newly trained group of interviewers. The interrater reliabilities of seven raters for 9 methadone maintenance (MM) pilot subjects were determined. These were found to be excellent for the CSs, but only moderate for the ISRs. Regression analyses were performed on the data of 407 methadone maintenance patients entering all of the variables in each area as the independent variables and the ISR as the dependent variable. These analyses indicated that, on average, 55-60% of the variance in ISRs was explained. However, while 63% of the variance was explained for the legal scale only 38% of the variance of the drug scale was explained. The subject's rating of either the need for treatment or the seriousness of the problem accounted for the most variance, with the exception of the drug and alcohol areas. The internal consistency of the composite scores was then examined for this sample using Cronbach's standardized alpha statistic. These were found to be generally satisfactory ranging from 0.62 for the drug scale to 0.87 for the alcohol and psychiatric scales. A mean interitem correlation of 0.11 for the drug scale was obtained suggesting relatively low item homogeneity. Finally, correlations between the CSs and ISRs were calculated for each scale. Moderate to high relationships were found (0.53-0.78) with the exception of the correlation for the employment area which was only 0.08. The findings are discussed in terms of the nature of and limitations of the ISRs and CSs.

Demographic, medical history and sexual correlates of HIV seropositive methadone maintained women

Beginning in January 1989, consecutive female admissions to the ARTC MMTP Clinics in NYC were interviewed about their medical, drug, sexual and social experiences during 6 distinct historical years. Bloods were drawn and each sample tested for HIV via ELISA and Western Blot analysis. The data for 256 females was analyzed. The sample was predominantly Black (56%) and Hispanic (36%). Fifty-four percent (140) were between the ages of 31 and 40; 35% (91) were between the ages of 18 and 30; and 10% (27) were 41 or older. The majority, 179 (69%), had less than a high school education, while 79 (31%) had a high school education or greater. The seropositivity for this sample of females was 60.4%. Aside from the common types of illnesses often seen in gay men infected with the HIV virus (i.e., pneumonia, night sweats, sore throat and swollen glands) our sample of females presented with symptoms such as abnormal discharges from the vagina, infections or abscesses of the veins, kidney or bladder infections, bleeding from the bowels and hepatitis infections. The most commonly reported risk factors among our sample of HIV positive females were sharing injecting materials (38%); injecting drugs in the veins (37.2%); dividing an injection (24.3%); and blood transfusions (10.9%). Of our HIV positive females, 42 of 97 (43.3%) reported having sex with a man they shared needles with only one time so that having sex with a man who is potentially infected with the HIV virus only once may be enough for a female to seroconvert. One limitation of this data is that there is no knowledge of when the HIV positive women seroconverted. Some of the behaviors reported could be due to exposure to AIDS education, and not to the knowledge to their HIV serostatus.

Drug abuse treatment programs as centers for HIV-related research and treatment

The Observational Data Base (ODB) is a multicenter, longitudinal effort of the Community Programs for Clinical Research in AIDS (CPCRA) designed to collect HIV-related data on a large number of HIV-infected patients receiving primary care from community based physicians and health care groups. Over 400 patients from the Addiction Research and Treatment Corporation have enrolled in the ODB. Compared to the remainder of the CPCRA, ARTC ODB enrollees are more likely to be female, African American or Hispanic and to have injectable drug use and heterosexual contact with an injectable drug user as risk factors. The ARTC ODB patient profile closely resembles the fastest growing segments of the AIDS epidemic.

Medical disorders in a cohort of New York city drug abusers: much more than HIV disease

We evaluated the medical histories, drug abuse patterns, sexual behaviors, serological studies for syphilis and hepatitis B infection and other laboratory studies in 1780 patients enrolled in New York City drug treatment clinics in 1987. HIV serology was available for 168 patients. Nearly seventy-five percent had at least one medical disorder and 57% one abnormal laboratory parameter. A history of gonorrhea, hepatitis B infection, pneumonia, and anemia was reported in 28%, 23%, 21%, and 20.7% of the patients, respectively. Fifty-seven percent of 168 patients tested HIV seropositive and 16% of 1,780 patients were tuberculin reaction positive. Given the considerable prevalence of medical disorders, in which early identification and intervention is possible, serious consideration should be given to extend the scope of drug abuse services to include primary medical care services targeted at the medical sequelae of drug abuse, and medical disorders not directly associated with either HIV disease or drug abuse.

Relationship of proton uptake on the cytoplasmic surface and reisomerization of the retinal in the bacteriorhodopsin photocycle: an attempt to understand the complex kinetics of the pH changes and the N and O intermediates

In the bacteriorhodopsin photocycle the recovery of the initial BR state from the M intermediate occurs via the N and O intermediates. The molecular events in this process include reprotonation of the Schiff base and the subsequent uptake of a proton from the cytoplasmic side, as well as reisomerization of the retinal from 13-cis to all-trans. We have studied the kinetics of the intermediates and the proton uptake. At moderately low pH little of the N state accumulates, and the O state dominates in the reactions that lead from M to BR. The proton uptake lags behind the formation of O, suggesting the sequence N(0)<==>O(0) + H+ (from the bulk)-->O(+1)-->BR+H+ (to the bulk), where the superscripts indicate the net protonation state of the protein relative to BR. Together with a parallel study of ours at moderately high pH, these results suggest that the sequence of proton uptake and retinal reisomerization depends on pH: at low pH the isomerization occurs first and O accumulates, but at high pH the isomerization is delayed and therefore N accumulates. Although this model contains too many rate constants for rigorous testing, we find that it will generate most of the characteristic pH-dependent kinetic features of the photocycle with few assumptions other than pH dependency for protonation at the proton release and uptake steps.

Alcohol abuse prevention in African-American communities

Because of the devastating morbidity, mortality, and social dislocations associated with alcohol abuse in many African-American communities, the prevention of alcohol abuse represents a critical component of the public health agenda for African Americans. This article reviews African-American cultural issues pertinent to alcohol use and abuse and the information available regarding the excess medical and social complications experienced by African Americans secondary to alcohol abuse. Some recommendations are made for a specific research agenda for the prevention of alcohol abuse with relevance to African Americans.

Photoreaction of the N intermediate of bacteriorhodopsin, and its relationship to the decay kinetics of the M intermediate

Because the M photointermediate of recombinant T46V bacteriorhodopsin decays more rapidly and the N intermediate more slowly than in wild-type, the photoreaction of N could be examined without interference from M. We found that between pH 6 and 9 the photoproducts of N included both earlier suggested M-like intermediate and red-shifted R state. However, when the photoexcitation of N was at wavelengths below 500 nm the amount of M-like product decreased with increasing pH, and at pH 9 virtually only R was produced. In the dark, T46V contains an N-like conformer, in increasing amounts with increasing pH like wild-type but in 4-5 times greater concentrations. The photoreaction of this thermally produced state is much like that of the N intermediate. It is associated with the appearance of a slowly decaying M, but we calculate that under most conditions used to follow M in the wild-type photocycle the amount of N-like conformer, and therefore the amplitude of this slow component, will not be significant. The results confirm the suggestion [Fukuda & Kouyama (1992) Biochemistry 31, 11740-11747] that an M-like state is included among the photoproducts of N, but at the same time provide support to photocycle models in which the slow component of the biphasic M decay is attributed not to this secondary photoreaction or to a separate photocycle originating from a heterogeneous initial state, but to thermal equilibration between M and N in a single photocycle.

Estimated acid dissociation constants of the Schiff base, Asp-85, and Arg-82 during the bacteriorhodopsin photocycle

The pK(a) values of D85 in the wild-type and R82Q, as well as R82A recombinant bacteriorhodopsins, and the Schiff base in the D85N, D85T, and D85N/R82Q proteins, have been determined by spectroscopic titrations in the dark. They are used to estimate the coulombic interaction energies and the pK(a) values of the Schiff base, D85, and R82 during proton transfer from the Schiff base to D85, and the subsequent proton release to the bulk in the initial part of the photocycle. The pK(a) of the Schiff base before photoexcitation is calculated to be in effect only 5.3-5.7 pH units higher than that of D85; overcoming this to allow proton transfer to D85 requires about two thirds of the estimated excess free energy retained after absorption of a photon. The proton release on the extracellular surface is from an unidentified residue whose pK(a) is lowered to about 6 after deprotonation of the Schiff base (Zimanyi, L., G. Varo, M. Chang, B. Ni, R. Needleman, and J.K. Lanyi, 1992. Biochemistry. 31:8535-8543). We calculate that the pK(a) of the R82 is 13.8 before photoexcitation, and it is lowered after proton exchange between the Schiff base and D85 only by 1.5-2.3 pH units. Therefore, coulombic interactions alone do not appear to change the pK(a) of R82 as much and D85 only by 1.5-2.3 pH units. Therefore, coulombic interactions alone do not appear to change the pK(a) of R82 as much as required if it were the proton release group.