23-year-old woman • syncopal episode • sinus bradycardia • history of bipolar disorder • Dx?

► Syncopal episode ► Sinus bradycardia ► History of bipolar disorder.

High Level of Hepatitis B Core-Related Antigen Associated With Increased Risk of Hepatocellular Carcinoma in Patients With Chronic HBV Infection of Intermediate Viral Load.

BACKGROUND & AIMS: Chronic hepatitis B virus (HBV) infection is a risk factor for hepatocellular carcinoma (HCC). Serum levels of HB core-related antigen (HBcrAg) have been associated with active replication of HBV. We investigated whether HBcrAg levels are associated with development of HCC, especially in patients who do not require antiviral treatment. METHODS: We collected data from 2666 adults positive for hepatitis B surface antigen (HBsAg), infected with HBV genotypes B or C, and without liver cirrhosis, who had long-term follow-up at the National Taiwan University Hospital from 1985 through 2000. None of the patients received antiviral treatment during the follow-up. Baseline levels of HBV DNA, HBsAg, and HBcrAg were determined retrospectively and participants were followed for a mean of 15.95 years. The primary end point was an association between serum level of HBcrAg and HCC development. RESULTS: HCC developed in 209 patients in the cohort (incidence rate, 4.91 cases/1000 person-years). We found a positive association between baseline level of HBcrAg and HCC development; HBcrAg level was an independent risk factor in multivariable analysis. In the subgroup of hepatitis B e antigen-negative patients with HBV DNA levels from 2000 to 19,999 IU/mL (intermediate viral load [IVL]) and normal levels of alanine aminotransferase, HBcrAg levels of 10 KU/mL or more identified patients at increased risk of HCC (hazard ratio, 6.29; confidence interval, 2.27-17.48). Patients with an IVL and a high level of HBcrAg had a risk for HCC that did not differ significantly from that of patients with a high viral load (≥20,000 IU/mL). Patients with an IVL but a low level of HBcrAg had a low risk of HCC, with an annual incidence rate of 0.10% (95% confidence interval, 0.04%-0.24%). CONCLUSIONS: In a long-term follow-up study of 2666 patients with chronic HBV infection (genotypes B or C), level of HBcrAg is an independent risk factor of HCC. Moreover, HBcrAg level of 10 KU/mL identifies patients with an IVL who are at high risk for HCC.

Why do not all living kidney donor candidates proceed to donation?

There are national and international guidelines for donor workup and acceptance criteria of potential living kidney donor candidates (LKDC), but there is significant variation in clinical practice. We examined our local practice in assessing potential LKDC against current guidelines; nearly all of our accepted donors met these guidelines. LKDC who did not proceed to donation had an identified health issue (60%), the presence of risk factors for long-term end-stage kidney disease (17%), social (13%) or immunological reasons (7%).

Fibrosis-4 Index Helps Identify HBV Carriers With the Lowest Risk of Hepatocellular Carcinoma.

OBJECTIVES: Several viral and host risk factors have been used to predict risks of hepatocellular carcinoma (HCC) in patients with chronic infection of hepatitis B virus (HBV). However, little is known whether fibrosis-4 (FIB-4) index, a liver fibrosis biomarker, helps identify non-cirrhotic patients with the lowest HCC risk. METHODS: A total of 2075 treatment-naive Taiwanese patients with chronic HBV infection were followed for an average period of 16.02 years. None of them had liver cirrhosis at baseline. We explored whether a low FIB-4 index complements the favourable predictors to defines patients with the lowest HCC risk. The finding was validated in 532 non-cirrhotic patients receiving long-term nucleos(t)ide analogue (NUC) treatment with suppressed viral replication. RESULTS: A total of 137 treatment-naive and 10 NUC-treated patients developed HCC, respectively. We found that HCC risk started to increase when baseline FIB-4 index >1.29 in the treatment-naive cohort. Patients with FIB-4 >1.29, compared to those with FIB-4 <1.29, were associated with a higher risk of HCC with hazards ratio of 5.56 (95% confidence interval: 3.93-7.86). More importantly, among patients with low viral load (HBV DNA level <2,000 IU/ml), baseline FIB-4 index helped stratify different HCC risks such that none of 326 HBeAg-negative patients with FIB-4 index <1.29, ALT level <40 U/l, and HBsAg level <1,000 IU/ml developed HCC. In addition, the patients with the FIB-4 index <1.29 consistently had the lowest HCC risks in the validation cohort receiving long-term NUC treatment. CONCLUSIONS: In non-cirrhotic patients with chronic HBV infection, FIB-4 index <1.29 complements the existing clinical profile to define patients with the lowest HCC risk.

Hepatitis B Surface Antigen Loss and Hepatocellular Carcinoma Development in Patients With Dual Hepatitis B and C Infection.

Hepatitis B virus (HBV) and hepatitis C virus (HCV) infections are 2 major causes of chronic viral hepatitis. It is still unclear how HCV coinfection affects HBV replication and clinical outcomes in HBV/HCV coinfected patients.We conducted a longitudinal study, which enrolled 111 patients with HBV/HCV coinfection and 111 propensity score-matched controls with HBV monoinfection. Both groups had comparable baseline age, sex, fibrosis stage, levels of HBV DNA, and HBV surface antigen (HBsAg). The HCV coinfection and other host/viral factors were correlated with various outcomes, including HBsAg loss and cirrhosis/hepatocellular carcinoma (HCC) development.After a 10-year follow-up, we found that HCV coinfection itself was not associated with HBsAg loss. However, coinfected patients with alanine aminotransferase (ALT) level >80 U/L had a higher chance of HBsAg loss than those with ALT level ≤80 U/L [hazard ratio (95% confidence interval): 4.41 (1.75-11.15)] or matched controls with HBV monoinfection [hazard ratio (95% confidence interval): 3.40 (1.54-7.50)]. Besides, both HCV coinfection and higher ALT levels were associated with higher HCC risks and the HCC risks remained even after HBsAg loss in HBV/HCV con-infected patient.HCV coinfection is not associated with HBsAg loss. A higher ALT level is a major determinant of HBsAg loss in patients with HBV/HCV coinfection. Both HCV coinfection and a higher ALT level were independent risk factors of HCC.

Higher proportion of viral basal core promoter mutant increases the risk of liver cirrhosis in hepatitis B carriers.

BACKGROUND AND OBJECTIVE: Precore (PC) variant (G1896A) and basal core promoter (BCP) variant (A1762T/G1764A) of HBV are associated with risk of hepatocellular carcinoma in HBV carriers. However, little is known about their impact on the adverse outcomes of hepatitis B e antigen (HBeAg)-negative hepatitis and liver cirrhosis. METHODS: 251 spontaneous HBeAg seroconverters who had genotype B or C infection and received a long-term follow-up were enrolled. PC and BCP mutants were determined qualitatively and quantitatively to correlate with these adverse outcomes. The findings were validated by an independent case-control study, which included 184 patients with biopsy-proven liver fibrosis stages. RESULTS: In the longitudinal cohort study, BCP mutant and possibly PC wild type were associated with cirrhosis development, but not HBeAg-negative hepatitis. Multivariable analysis showed that only BCP mutant was an independent risk factor for cirrhosis development. Using quantitative analysis of BCP mutant, a higher proportion of BCP mutant, defined as a continuous variable, a dichotomous variable or an ordinal variable, was associated with a higher risk of cirrhosis. If we chose 45% of BCP mutant as the cut-off, the risk of cirrhosis was higher in patients with BCP mutant ≥45% compared to <45% in the longitudinal cohort; this finding was validated by the case-control study (adjusted OR: 2.81, 95% CI 1.40 to 5.67). CONCLUSIONS: A higher proportion of BCP mutant increases the risk of liver cirrhosis development in HBV carriers with genotype B or C infection.

Hepatitis B surface antigen level complements viral load in predicting viral reactivation in spontaneous HBeAg seroconverters.

BACKGROUND AND AIMS: The level of hepatitis B surface antigen (HBsAg) has been shown to complement hepatitis B virus (HBV)-DNA level in predicting disease progression in hepatitis B e antigen (HBeAg)-negative patients, especially those with low viral loads. Whether this finding could be seen in spontaneous HBeAg seroconverters remains unclear. METHODS: A retrospective cohort of 390 Taiwanese spontaneous HBeAg seroconverters with a mean follow-up period of 7.4 years was enrolled. The relationships between HBV-DNA/HBsAg levels and HBeAg-negative hepatitis/active viral replication (HBV-DNA level ≥ 2000 IU/mL) were investigated. RESULTS: In the overall cohort, serum HBV-DNA level served as a better predictor for HBeAg-negative hepatitis compared with HBsAg level. However, in those with HBV-DNA level < 2000 IU/mL, a higher HBsAg level was associated with a higher risk of HBeAg-negative hepatitis (P = 0.015). Multivariate analysis showed the hazard ratio of HBsAg level ≥ 1000 IU/mL versus < 1000 IU/mL was 4.1 (95% confidence interval: 1.3-13.6). When using the end-point of active viral replication, HBsAg ≥ 1000 IU/mL remained as an independent risk factor, with a hazard ratio of 2.5 (95% confidence interval: 1.1-5.9). CONCLUSIONS: In spontaneous HBeAg seroconverters with HBV-DNA level < 2000 IU/mL, HBsAg level ≥ 1000 IU/mL is associated with increased risks of HBeAg-negative hepatitis and active viral replication. Combining HBV-DNA < 2000 IU/mL and HBsAg level < 1000 IU/mL may be used to define minimal viral activity.

Risk stratification of hepatocellular carcinoma in hepatitis B virus e antigen-negative carriers by combining viral biomarkers.

BACKGROUND: The serum hepatitis B virus (HBV) surface antigen (HBsAg) level can predict hepatocellular carcinoma (HCC) development in hepatitis B e antigen (HBeAg)-negative patients with an HBV DNA level of <2000 IU/mL. However, little is known regarding how well the combination of both viral biomarkers stratifies HCC risk. METHODS: A total of 2165 Taiwanese HBeAg-negative noncirrhotic patients were followed for 14.9 years. The predictive power of the HBsAg level for HCC was analyzed for different viral load ranges. RESULTS: In patients with HBV DNA levels of 2000-19 999 IU/mL (intermediate viral load), a positive correlation between HBsAg level and HCC development was identified after adjustment for other risk factors (P = .002). In contrast, no association was found between HBsAg level and HCC in patients with higher viral loads. HBsAg level was subsequently included to stratify HCC risk in patients with low and intermediate viral loads. Receiver operating characteristic curve analysis showed that combining HBV DNA and HBsAg level better predicts 10-year HCC development as compared to using HBV DNA level alone in the overall cohort (P = .028). CONCLUSIONS: Serum HBsAg level helps stratify HCC risk in patients with intermediate viral loads. Combining HBV DNA and HBsAg levels better predicts HCC risk.

Serum hepatitis B surface antigen levels help predict disease progression in patients with low hepatitis B virus loads.

UNLABELLED: Chronic hepatitis B patients with high viral loads are at increased risk of cirrhosis and hepatocellular carcinoma (HCC). In patients with low viral loads, higher hepatitis B surface antigen (HBsAg) levels have been shown to predict HCC development. However, little is known about the difference in risk for other hepatitis B virus (HBV)-related adverse outcomes with varying HBsAg levels. A total of 1,068 Taiwanese hepatitis B e antigen (HBeAg)-negative HBV carriers with serum HBV DNA level <2,000 IU/mL at baseline were followed for a mean duration of 13.0 years. Patients were categorized based on their HBsAg levels, and the relationships between HBsAg level and development of HBeAg-negative hepatitis, hepatitis flare, and cirrhosis were investigated. Of the 1068 patients with low viral loads, 280 developed HBeAg-negative hepatitis, with an annual incidence rate of 2.0%. HBsAg level, but not HBV DNA level, was found to be a risk factor for HBeAg-negative hepatitis. Multivariate analysis showed that the adjusted hazard ratio in patients with an HBsAg level ≥ 1,000 versus <1000 IU/mL was 1.5 (95% confidence interval, 1.2-1.9). The positive correlation was present when evaluating other endpoints, including hepatitis flare and cirrhosis, and remained consistent when the study population was restricted to those with normal alanine aminotransferase (ALT) level at baseline. The annual incidence rate of HBeAg-negative hepatitis was lowered to 1.1% in patients with low levels of HBV DNA, HBsAg, and ALT. CONCLUSION: In HBeAg-negative patients with low viral loads and genotype B or C virus infection, a higher HBsAg level can predict disease progression. HBsAg <1,000 IU/mL in combination with low levels of HBV DNA and ALT help define minimal-risk HBV carriers.

Young chronic hepatitis B patients with nucleos(t)ide analogue-induced hepatitis B e antigen seroconversion have a higher risk of HBV reactivation.

BACKGROUND: It is unclear whether hepatitis B e antigen (HBeAg) seroconversion induced by nucleos(t)ide analogues (NUC) has a prognosis that is similar to that of spontaneous HBeAg seroconversion. METHODS: A total of 148 noncirrhotic NUC-induced HBeAg seroconverters were consecutively enrolled. A historical control of 407 noncirrhotic spontaneous HBeAg seroconverters was also recruited. We compared the rates of HBeAg seroreversion and HBV reactivation between these 2 cohorts. RESULTS: There were 1652.8 and 465.2 person-years of follow-up for spontaneous and NUC-induced HBeAg seroconverters, respectively. Compared with NUC-induced seroconverters, spontaneous seroconverters were younger when achieving HBeAg seroconversion. We thus compared these 2 cohorts according to their age at HBeAg seroconversion. In patients achieving HBeAg seroconversion before 30 years of age, NUC-induced seroconverters had a higher 2-year HBeAg seroreversion rate than spontaneous seroconverters (12.0% vs 2.9%; P = .004) and were at a higher risk of HBV reactivation (hazard ratio, 4.6; 95% confidence interval, 1.5-14.4). Using multivariate analysis, NUC-induced HBeAg seroconversion remained a risk factor of both endpoints in young HBeAg seroconverters. CONCLUSION: NUC-induced HBeAg seroconverters may not have durable response after stopping therapy. For patients achieving HBeAg seroconversion before 30 years of age, the risk of HBeAg seroreversion and HBV reactivation is higher in NUC-induced seroconverters than spontaneous HBeAg seroconverters.

High levels of hepatitis B surface antigen increase risk of hepatocellular carcinoma in patients with low HBV load.

BACKGROUND & AIMS: Patients with chronic hepatitis B virus (HBV) infection have a high risk for developing hepatocellular carcinoma (HCC). Patients with lower levels of hepatitis B surface antigen (HBsAg) have higher chances of losing HBsAg than those with high levels. However, little is known about whether higher levels of HBsAg increase risk for HCC. METHODS: We followed 2688 Taiwanese HBsAg-positive patients without evidence of cirrhosis for a mean time period of 14.7 years. In addition to the known risk factors of HCC, we investigated the association between levels of HBsAg and development of HCC. RESULTS: Of the patients followed, 191 developed HCC, with an average annual incidence rate of 0.5%. Baseline levels of HBsAg and HBV were associated with development of HCC, and risk increased with level. Compared to HBsAg level, by receiver operating characteristic curve analysis, HBV DNA level better predicted the development of HCC during 10-year and 15-year periods (both, P < .001). However, when we evaluated hepatitis B e antigen-negative patients with levels of HBV DNA <2000 IU/mL, factors that determined HCC risk included sex, age, and levels of alanine aminotransferase and HBsAg (≥1000 IU/mL), but not level of HBV DNA. Multivariate analysis showed that the adjusted hazard ratio for HCC in patients with levels of HBsAg ≥1000 IU/mL versus <1000 IU/mL was 13.7 (95% confidence interval: 4.8-39.3). CONCLUSIONS: Among patients infected with HBV genotype B or C, determinants of HCC risk include their sex, age, hepatitis B e antigen status, HBV genotype, and levels of alanine aminotransferase and HBV DNA, but not level of HBsAg. Among hepatitis B e antigen-negative patients with low viral loads, HCC risk is determined by levels of HBsAg and alanine aminotransferase and age, but not HBV DNA.

Determinants of spontaneous surface antigen loss in hepatitis B e antigen-negative patients with a low viral load.

UNLABELLED: Loss of hepatitis B surface antigen (HBsAg) usually indicates the cure of hepatitis B virus (HBV) infection. In spontaneous hepatitis B e antigen (HBeAg) seroconverters, lower serum HBsAg and HBV DNA levels have been shown to be associated with HBsAg loss over time. However, little is known about their impacts on HBsAg loss in HBeAg-negative patients with limited viral replication. A total of 688 HBeAg-negative patients with baseline serum HBV DNA levels <2000 IU/mL were enrolled in Taiwan. The relationships of HBsAg and HBV DNA levels with subsequent HBsAg loss were investigated. In a mean follow-up of 11.6 years, the average annual rate of HBsAg loss was 1.6%. Baseline HBsAg and HBV DNA levels were inversely associated with subsequent HBsAg loss. When compared to patients who had HBsAg levels >1000 IU/mL, the rates of HBsAg loss were significantly higher in patients with HBsAg levels of 100-999, 10-99, and <10 IU/mL, with hazard ratios of 2.5 (95% confidence interval [CI], 1.6-4.0), 2.8 (95% CI, 1.6-5.0), and 13.2 (95% CI, 8.1-21.5), respectively. Multivariate analysis showed that HBsAg level, but not HBV DNA, remained as an independent factor. The adjusted hazard ratio of HBsAg loss was 13.2 (95% CI, 7.8-22.1) for HBsAg level <10 versus ≥ 1000 IU/mL. When compared to HBV DNA level by receiver operating characteristic curve analysis, HBsAg level served as a better predictor of both 5-year and 10-year HBsAg loss. CONCLUSION: In HBeAg-negative patients with HBV genotype B or C infection who have HBV DNA level <2000 IU/mL, HBsAg level <10 IU/mL is the strongest predictor of HBsAg loss.

Serum interleukin 6 level correlates with outcomes of acute exacerbation of chronic hepatitis B.

PURPOSE: Acute exacerbation (AE) of chronic hepatitis B virus (HBV) infection is common and negatively impacts the clinical outcome. Factors predicting outcomes after exacerbations were only partly clarified. We investigated the host immune parameters associated with long-term outcomes. METHODS: We prospectively examined the profiles of serum cytokines and chemokines in 36 consecutive hepatitis B e antigen (HBeAg)-positive patients (male 72%, age 40.8 ± 9.9 years, genotype B/C 75%/25%) who developed AE in a medical center. The patients were followed up for a median of 4 years (range 2-6 years) post-AE. The impact of six cytokines (tumor necrosis factor alfa, interferon gamma, IL-2, IL-4, IL-6, and IL-10) and five chemokines (CXCL10/IP-10, CCL2/MCP-1, CXCL9/MIG, CCL5/RANTES, and CXCL8/IL-8) at the onset of AE activity on the long-term outcomes were analyzed. RESULTS: Of 36 patients, 22 (61.1%) developed HBeAg seroconversion during follow-up (Group I), and the remaining 14 patients did not obtain HBeAg seroconversion (Group II). Baseline characteristics were generally similar between two groups of patients. In Group I patients, the frequency of undetectable serum IL-6 level (<3 pg/mL) at the onset of AE was significantly higher in comparison with Group II patients in multivariate analysis (86.4 vs. 42.9%, P = 0.016). CONCLUSIONS: Our findings indicate that undetectable serum IL-6 level at the early stage of AE correlated with the long-term outcomes and may serve as a useful clinical predictor.

Serum hepatitis B virus-DNA levels correlate with long-term adverse outcomes in spontaneous hepatitis B e antigen seroconverters.

BACKGROUND: Hepatitis B e antigen (HBeAg) status and serum hepatitis B virus (HBV) DNA levels are major factors affecting the prognosis of adult HBV carriers; however, the impact of viral load on long-term outcomes after spontaneous HBeAg seroconversion remains unclear. METHODS: A total of 390 spontaneous HBeAg seroconverters with a long-term follow-up were enrolled. Serum HBV-DNA levels at 1 year after HBeAg seroconversion were determined, and their correlation with long-term adverse outcomes was explored. RESULTS: In a mean follow-up of 6.8 years, the average annual incidence rates were 4.4% and 1.9% for HBeAg-negative hepatitis and hepatitis flare, respectively. Compared with patients with HBV-DNA levels <200 IU/mL, the adjusted hazard ratios of HBeAg-negative hepatitis were 2.4 (95% confidence interval, 1.3-4.4), 3.6 (1.8-7.2), and 5.3 (2.8-10.0), respectively, for serum HBV-DNA level of 2000 -2 × 10(4), 2 × 10(4) -2 × 10(5), and ≥2 × 10(5) IU/mL. In addition, serum HBV-DNA levels were independently associated with HBeAg-negative hepatitis flare, which confirmed their impact on the immune active hepatitis after HBeAg seroconversion. CONCLUSIONS: HBeAg seroconversion may not always confer favorable outcomes. Serum HBV-DNA levels ≥2000 IU/mL at 1 year post HBeAg seroconversion correlate with increased risk of HBeAg-negative hepatitis and hepatitis flare.

Effect of host and viral factors on hepatitis B e antigen-positive chronic hepatitis B patients receiving pegylated interferon-α-2a therapy.

BACKGROUND: Pegylated interferon (PEG-IFN)-α-2a improves the hepatitis B e antigen (HBeAg) seroconversion rate in HBeAg-positive chronic hepatitis B patients. However, baseline factors predicting favourable responses to PEG-IFN-α-2a remain largely unknown. METHODS: A total of 115 HBeAg-positive chronic hepatitis B patients who had a pre-therapy serum alanine aminotransferase (ALT) level over two times the upper limit of normal and received PEG-IFN-α-2a for 6-12 months were consecutively enrolled according to the local reimbursed guidelines. HBeAg seroconversion and combined response defined as HBeAg seroconversion, HBV-DNA level <20,000 IU/ml as well as ALT normalization at 6 months off therapy were primary and secondary therapeutic end points, respectively. Baseline viral factors, including viral load, genotype and major sequences of precore stop codon/basal core promoter (BCP), and host factors, including three single nucleotide polymorphisms among the HLA-DPA1, HLA-DPB1 and IL28B regions, were determined to correlate with therapeutic end points. RESULTS: HBeAg seroconversion and combined response rates were 26.1% and 18.3%, respectively. By multivariate analysis, BCP mutation (OR 8.04, 95% CI 2.00-32.28) and rs3077 G/G genotype (OR 3.49, 95% CI 1.12-10.84) were associated with a higher HBeAg seroconversion rate; BCP mutation (OR 9.28, 95% CI 1.92-44.99) and baseline viral load <2 × 10(6) IU/ml (OR 4.78, 95% CI 1.37-16.69) were associated with a higher combined response rate. CONCLUSIONS: BCP mutation is associated with higher HBeAg seroconversion and combined response rates at 6 months off therapy in HBeAg-positive chronic hepatitis B patients treated with PEG-IFN-α-2a. Genetic variants in the HLA-DPA1 region may also affect treatment-induced HBeAg seroconversion.

Aromatic lipophilic spin traps effectively inhibit RPE65 isomerohydrolase activity.

We previously showed that RPE65 does not specifically produce 11-cis-retinol only but also 13-cis-retinol, supporting a carbocation or radical cation mechanism of isomerization. The intrinsic properties of conjugated polyene chains result in facile formation of radical cations in oxidative conditions. We hypothesized that such radical intermediates, if involved in the mechanism of RPE65, could be stabilized by spin traps. We tested a variety of hydrophilic and lipophilic spin traps for their ability to inhibit RPE65 isomerohydrolase activity. We found that the aromatic lipophilic spin traps such as N-tert-butyl-α-phenylnitrone (PBN), 2,2-dimethyl-4-phenyl-2H-imidazole-1-oxide (DMPIO), and nitrosobenzene (NB) strongly inhibit RPE65 isomerohydrolase activity in vitro.

RPE65, visual cycle retinol isomerase, is not inherently 11-cis-specific: support for a carbocation mechanism of retinol isomerization.

The mechanism of retinol isomerization in the vertebrate retina visual cycle remains controversial. Does the isomerase enzyme RPE65 operate via nucleophilic addition at C(11) of the all-trans substrate, or via a carbocation mechanism? To determine this, we modeled the RPE65 substrate cleft to identify residues interacting with substrate and/or intermediate. We find that wild-type RPE65 in vitro produces 13-cis and 11-cis isomers equally robustly. All Tyr-239 mutations abolish activity. Trp-331 mutations reduce activity (W331Y to approximately 75% of wild type, W331F to approximately 50%, and W331L and W331Q to 0%) establishing a requirement for aromaticity, consistent with cation-pi carbocation stabilization. Two cleft residues modulate isomerization specificity: Thr-147 is important, because replacement by Ser increases 11-cis relative to 13-cis by 40% compared with wild type. Phe-103 mutations are opposite in action: F103L and F103I dramatically reduce 11-cis synthesis relative to 13-cis synthesis compared with wild type. Thr-147 and Phe-103 thus may be pivotal in controlling RPE65 specificity. Also, mutations affecting RPE65 activity coordinately depress 11-cis and 13-cis isomer production but diverge as 11-cis decreases to zero, whereas 13-cis reaches a plateau consistent with thermal isomerization. Lastly, experiments using labeled retinol showed exchange at 13-cis-retinol C(15) oxygen, thus confirming enzymatic isomerization for both isomers. Thus, RPE65 is not inherently 11-cis-specific and can produce both 11- and 13-cis isomers, supporting a carbocation (or radical cation) mechanism for isomerization. Specific visual cycle selectivity for 11-cis isomers instead resides downstream, attributable to mass action by CRALBP, retinol dehydrogenase 5, and high affinity of opsin apoproteins for 11-cis-retinal.