Short-term anti-TNF therapy with surgical closure versus anti-TNF therapy alone for Crohn's perianal fistulas (PISA-II): long-term outcomes of an international, multicentre patient preference, randomised controlled trial

Background

The PISA-II trial showed that short-term anti-tumour necrosis factor (anti-TNF) therapy followed by surgical closure induces radiological healing of perianal fistulas in patients with Crohn's disease more frequently than anti-TNF therapy alone after 18 months. This study aimed to compare long-term outcomes of both treatment arms.

Methods

Follow-up data were collected from patients who participated in the PISA-II trial, an international patient preference randomised controlled trial. This multicentre trial was performed in nine hospitals in the Netherlands and one hospital in Italy. Patients with Crohn's disease above the age of 18 years with an active high perianal fistula and a single internal opening were asked to participate. Patients were allocated to anti-TNF therapy (intravenous infliximab, or subcutaneous adalimumab, at the discretion of the gastroenterologist) for one year, or surgical closure combined with 4-months anti-TNF therapy. Patients without a treatment preference were randomised (1:1) using random block randomisation (block sizes of six without stratification), and patients with a treatment preference were treated according to their preferred treatment arm. For the current follow-up study, data were collected until May 2022. Primary outcome was radiological healing on magnetic resonance imaging (MRI), including all participants with a MRI made less than 6 months ago at the time of data collection. Analysis was based on observed data.

Findings

Between September 14, 2013, and December 7, 2019, 94 patients were enrolled in the trial. Long-term follow-up data were available in 91 patients (36/38 (95%) anti-TNF + surgical closure, 55/56 (98%) anti-TNF). A total of 14/36 (39%) patients in the surgical closure arm were randomly assigned, which was not significantly different in the anti-TNF treatment arm (16/55 (29%) randomly assigned). Median follow-up was 5.7 years (interquartile range (IQR) 5-7). Radiological healing occurred significantly more often after anti-TNF + surgical closure (15/36 = 42% versus 10/55 = 18%; P = 0.014). Clinical closure was comparable (26/36 = 72% versus 34/55 = 62%; P = 0.18) in both groups. However, clinical closure in the surgical group was achieved with less re-interventions 4/26 (= 15%) versus 18/34 (= 53%), including (redo-)surgical closure procedures. Recurrences occurred in 0/25 (0%) patients with radiological healing versus 27/76 (36%) patients with clinical closure, sometime during follow-up. Anti-TNF trough levels were higher in patients with long-term clinical closure in both groups (P = 0.031 and P = 0.014). In 6/11 (55%) patients in the anti-TNF group with available trough levels, recurrences were diagnosed within three months of a drop under 3.5ug/ml. 36 patients stopped anti-TNF, after which 0/14 (0%) patients with radiological healing developed a recurrence and 9/22 (41%) with clinical closure. Self-rated (in)continence was comparable between groups, and 79% (60/76) of patients indicated comparable/improved continence after treatment. Decision-regret analysis showed that all (30/30) anti-TNF + surgical closure patients agreed or strongly agreed that surgery was the right decision versus 78% (36/46) in the anti-TNF arm. All surgical closure patients would go for the same treatment again, whereas this was 89% (41/46) in the anti-TNF arm.

Interpretation

This study confirmed that surgical closure should be considered in amenable patients with perianal fistulas and Crohn's disease as long-term outcomes were favourable, and that radiological healing should be the aim of treatment as recurrences only occurred in patients without radiological healing. In patients with complete MRI closure, anti-TNF could be safely stopped.

Funding

None.

Inflammation: a possible mechanism for a causative role of hyperuricemia/gout in cardiovascular disease

Hyperuricemia and gout are independent risk factors associated with the development of hypertension, metabolic syndrome, vascular damage, and renal disease. Whether these risk factors are causally related to these important chronic co-morbidities remains uncertain, but inflammation may provide a mechanistic explanation. Hyperuricemia and gout negatively affect vascular function by exerting pro-oxidant effects and by decreasing nitric oxide bioavailability, thus inducing inflammation and endothelial dysfunction, which may promote hypertension, metabolic syndrome, and cardiovascular (CV) disease. This paper presents and discusses current understanding of the diverse influences promoting hyperuricemia and gout and the basis of acute and chronic hyperuricemia/gout-related inflammation. This review is based on a PubMed/Embase database search for articles on hyperuricemia and its impact on cardiovascular and renal function.

Diabetes and gout: efficacy and safety of febuxostat and allopurinol

AIM

Assess influences of demographics and co-morbidities of gout patients with or without diabetes on safety and efficacy of urate-lowering agents.

METHODS

Post-hoc analysis of 312 diabetic and 1957 non-diabetic gout patients [baseline serum urate levels (sUA) ≥8.0 mg/dl] enrolled in a 6-month randomized controlled trial comparing urate-lowering efficacy (ULE) and safety of daily xanthine oxidase inhibitors (XOIs) febuxostat (40 mg or 80 mg) and allopurinol (200 mg or 300 mg). We compared baseline demographic, gout and co-morbid characteristics, ULE, and safety of XOI treatment in diabetic and non-diabetic gout patients. ULE was measured by the proportion of diabetic and non-diabetic patients in each treatment group achieving final visit sUA < 6.0 mg/dl. Safety was monitored throughout the trial.

RESULTS

Diabetic gout patients were older, more frequently female, and had longer gout duration. Co-morbidities were more frequent among diabetic patients: cardiovascular disease; impaired renal function; hyperlipidemia; and obesity (body mass index >30 kg/m²) (p < 0.001 for all comparisons). Febuxostat 80 mg ULE exceeded that of febuxostat 40 mg or allopurinol (p < 0.050) at all levels of renal function, achieving sUA goal range in the majority of diabetic and non-diabetic patients. Diabetics and non-diabetics reported self-limiting diarrhoea and URIs as the most common adverse events.

CONCLUSIONS

Despite higher co-morbidity rates in diabetic patients, febuxostat and allopurinol were safe in both groups at the doses tested. Febuxostat 80 mg achieved sUA <6.0 mg/dl more often than febuxostat 40 mg or allopurinol at commonly prescribed doses.

P2-03-03: An Insulin-Like Growth Factor I (IGF-I)-Induced Gene, Solute Carrier Family 7 Member 11 (SLC7A11)/xCT, Mediates IGF-I-Induced Biological Behaviors in Breast Cancer Cells

Our laboratory studied the gene expression profiles of a series of T47D variant cell lines with differential insulin receptor substrate (IRS) adaptor protein expression to develop predictive IGF-I pathway biomarkers. We identified an IGF-I-induced gene, SLC7A11 (or xCT), which is specifically regulated through IRS-1. xCT encodes the cystine/glutamate transporter subunit of the heterodimeric amino acid transport system xc- which is a major plasma membrane transporter for the cellular uptake of cystine in exchange for intracellular glutamate. xCT is involved in the regulations of proliferation, metastasis, and drug resistance in various cancers. However, to date, the linkage between xCT and the IGF-I signaling pathway has not been described. To study the role for xCT in mediating IGF-I-induced biology in breast cancer cell lines, we examined xCT mRNA expression upon IGF-I stimulation in two breast cancer cell lines; the MCF-7 (IRS-1 activated) and MDA-MB-231 (IRS-2 activated) cells. Significant increased xCT expression was observed only in MCF-7 cells after IGF-I treatment. Immunoblots showed that xCT protein expression was elevated after IGF-I treatment and induced glutamate/cystine exchange in MCF-7 cells. shRNA was used to downregulate xCT in MCF-7 and MDA-MB-231 cells. In MCF-7, IGF-I-stimulated cell monolayer growth was suppressed by xCT shRNA or by the xCT inhibitor sulfasalazine (SASP). In MDA-MB-231 cells, xCT downregulation did not affect IGF-mediated Boyden chamber migration. Thus, IGF-I induction of cellular xCT levels is associated with cell growth in the IRS-1 activated MCF-7 cells, while MDA-MB-231 cells were not affected by downregulation of this gene. Therefore, our data imply that xCT may mediate IGF-I induced biological functions in breast cancer cell lines through an IRS-1 dependent pathway.

Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr P2-03-03.

P2-11-11: IGFBP Ratio Confers Resistance to IGF Targeting and Correlates with Increased Invasion and Poor Outcome in Breast Tumors

Purpose: To assess the role of insulin-like growth factor binding protein 5 (IGFBP-5) as a marker of relapse and survival in breast cancer tumors.

Experimental Design: Targeted regulation of IGFBP-5 was identified in MCF-7 cells resistant (MCF-7R4) to the IGF-1R/InsR inhibitor, BMS-536924 and examined by comparative microarray analysis, western and ELISA. Protein expression of IGFBP-5 was measured by immunohistochemistry in a cohort of 84 breast cancer patients to examine correlative associations with invasive tumor fraction and overall survival (OS). The expression ratio of IGFBP-5/IGFBP-4 (BPR) was determined in multiple breast tumor cohorts for univariate analysis.

Results: IGFBP-5 was markedly upregulated and highly localized to the membrane in MCF-7R4 resistant cells. When compared to pathologically normal reduction mammoplasty tissue, IGFBP-5 expression levels were upregulated in both invasive and histologically normal adjacent breast cancer tissue. In an independent cohort of breast cancer patients, IGFBP-5 protein levels correlated directly with invasion and OS. In univariate and multivariate modeling, metastasis-free survival, recurrence free survival (RFS) and OS were significantly associated with high IGFBP-5 expression. Prognostic power of IGFBP-5 was further increased with the addition of IGFBP-4 and tumors were ranked based upon IGFBP-5/IGFBP-4 expression ratio (BPR). Multiple breast cancer cohorts confirm that BPR (high vs. low) was a strong predictor of RFS and OS.

Conclusion: IGFBP-5 expression is a marker of poor outcome in breast cancer patients. An IGFBP-5/IGFBP-4 expression ratio may serve as a surrogate biomarker of IGF pathway activation and predict sensitivity to IGF-1R-targeted therapies.

Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr P2-11-11.

Work productivity loss due to flares in patients with chronic gout refractory to conventional therapy

OBJECTIVE

Joint pain and swelling during gout flares may lead to considerable morbidity and disability, having an impact on patient work productivity and social participation. The objective of this study was to assess how gout flares affect these activities in patients with chronic gout refractory to conventional therapy.

METHODS

A 1-year prospective observational study was conducted among patients with symptomatic disease in the United States in 2001. Inclusion criteria required patients (1) to be age 18 years or older, (2) to have documented, crystal-proven gout, (3) to have symptomatic gout, and (4) to be intolerant or unresponsive to conventional therapy, reflected by SUA ≥ 6.0 mg/dL. Patients were evaluated every 2 months. At each visit, patients completed a gout diary, which included number of flares experienced, duration and severity of each flare, and whether the flare caused: (1) work loss, (2) missed appointments or social events, or (3) impairment of self-care activities. The Short-Form Health Survey (SF-36) was also completed each visit.

RESULTS

Analyses were restricted to those who completed the first 6 months of the study (n = 81). Mean number of flares per patient per year was 8.8. Of the patients who were <65 years, 78% reported at least 1 work day lost due to a gout attack during the year. Mean annual work day loss for those <65 years was 25.1 days. A total of 545 of patients reported at least one flare per year that impaired social activities, with a mean of 17.1 social days lost and 52% reported at least one flare per year that compromised normal self-care activities, with a mean of 16.9 days impairment. Correlations between the diary reports and activity-related questions from the SF-36 were significantly positive.

LIMITATIONS

The study is limited by small sample size, lack of reference group, and inability to explicitly collect employment information. Age under 65 years was used as a proxy for employment eligibility.

CONCLUSION

Flares in patients with chronic gout refractory to conventional therapy significantly affect patient work productivity and social activities.

Febuxostat in the treatment of gout: 5-yr findings of the FOCUS efficacy and safety study

OBJECTIVES

This 5-yr study assessed urate-lowering and clinical efficacy and safety of long-term febuxostat therapy in subjects with gout. The primary efficacy end-point was reduction to and maintenance of serum urate (sUA) levels < 6.0 mg/dl.

METHODS

Subjects who completed a previous 28-day study were entered into an open-label extension study and initially received febuxostat 80 mg daily. Between Weeks 4 and 24, dosing could be adjusted to febuxostat 40 or 120 mg. All subjects received gout flare prophylaxis during the first 4 weeks. Gout flares were recorded and treated throughout the study, and sUA, baseline tophi and safety were monitored.

RESULTS

Among 116 subjects initially enrolled, dose adjustments were made for 44 (38%) subjects. As a result, 8 subjects received febuxostat 40 mg, 79 received 80 mg, and 29 received 120 mg daily maintenance dose. At 5 yrs, 93% (54/58) of the remaining subjects had sUA < 6.0 mg/dl. Fifty-eight subjects (50%) discontinued prematurely; 38 did so in the first year. Thirteen subjects withdrew due to an adverse event. Sustained reduction of sUA was associated with nearly complete elimination of gout flares. In 26 subjects with a tophus at baseline, resolution was achieved in 69% (18/26) by last visit on study drug at any point during the study (Final Visit). There were no deaths reported during the study.

CONCLUSIONS

Long-term treatment with febuxostat resulted in durable maintenance of sUA < 6.0 mg/dl for most subjects. There was nearly complete abolition of gout flares in patients completing the study. Baseline tophi resolved in a majority of subjects.

IGF-I mediated phosphorylation of ERαS167 occurs downstream of mTOR/p70S6K1 to impact nuclear localization and chromatin interaction

Abstract #6012

Crosstalk between the insulin-like growth factor (IGF) I receptor (IGF-IR) and estrogen receptor alpha (ERα) enhances breast cancer cell growth and survival. Interaction between these receptors has been extensively characterized in numerous in vitro and in vivo cell models. Activation of the IGF-IR results in the recruitment of the serine/threonine kinase Akt/PKB, which has been shown to directly phosphorylate ERα at Serine167. These findings were demonstrated in transient transfection model systems. To study if IGF-IR activation of Akt/PKB results in ERα phosphorylation, we used the MCF-7L breast cancer cell line expressing both IGF-IR and ERα. In these cells, IGF-I-induced activation of the IGF-IR/PI3K/Akt axis resulting in the subsequent phosphorylation of ERα at serine 167 as early as 10 minutes and was seen up to 24 hours indicating that this was not a transient phenomenon. In addition, this event was site-specific for ser167, as IGF-I exposure did not result in the phosphorylation of the estradiol (E2)/mitogen activated protein kinase (MAPK) regulated serine 118 site of ERα. Contrary to prior reports, we observed that IGF-I induced ERαS167 phosphorylation was abolished by rapamycin, an inhibitor of mammalian target of rapamycin (mTOR). As expected, rapamycin failed to block E2 stimulated ERαS118 phosphorylation. These events were further confirmed in two additional strains of MCF-7 cells (MVLN & MCF-7ATCC). In an effort to confirm that the events responsible for IGF-I induced ERαS167 phosphorylation were distal to mTOR activation, we targeted multiple additional signaling molecules in the IGF-IR/PI3K/Akt/mTOR pathway. Blockade of PI3K (LY294002), mTOR (rapamycin) and the downstream mTOR kinase p70S6K1 (H89) resulted in a dose-dependent ablation of ERαS167 phosphorylation. In addition to blocking ERα phosphorylation, H89 specifically also inhibited eukaryotic initiation factor (eIF) 4B (eIF4B), a known immediate downstream target of activated p70S6K1. Phosphorylation of ERαS118 did not require MAPK activation, as the MEK inhibitor U0126 ablated MAPK activation and did not affect ERαS167 phosphorylation. Furthermore, we find that siRNA-mediated knockdown of p70S6K1 results in a concordant attenuation of IGF-mediated ERαS167 phosphorylation independent of the ERαS118 site. Thus, we find that phosphorylation of ERαS167 in MCF-7L cells is regulated downstream of IGF-IR by p70S6K1, a kinase known to initiate the translation of multiple mRNA transcripts. Since modification of ERα by serine phosphorylation has been shown to impact DNA binding, transcription, dimerization and coactivator recruitment, we hypothesize that IGF-IR/PI3K/Akt initiation induces ERαS167 phosphorylation through an PI3K/mTOR-specific pathway and may alter classical nuclear ERα function. More importantly, IGF/ERα crosstalk occurs at the level of post-translational modification of ERα and may serve to promote the malignant phenotype in breast cancer cells.

Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 6012.

A modified Delphi exercise to determine the extent of consensus with OMERACT outcome domains for studies of acute and chronic gout

OBJECTIVES

To reach consensus with recommendations made by an OMERACT Special Interest Group (SIG).

METHODS

Rheumatologists and industry representatives interested in gout rated and clarified, in three iterations, the importance of domains proposed by the OMERACT SIG for use in acute and chronic gout intervention studies. Consensus was defined as a value of less than 1 of the UCLA/RAND disagreement index.

RESULTS

There were 33 respondents (61% response rate); all agreed the initial items were necessary, except "total body urate pool". Additional domains were suggested and clarification sought for defining "joint inflammation" and "musculoskeletal function". Items that demonstrated no clear decision were re-rated in the final iteration. There were six highly rated items (rating 1-2) with four slightly lower rating items (rating 3) for acute gout; and 11 highly rated items with eight slightly lower ratings for chronic gout.

CONCLUSIONS

Consensus is that the following domains be considered mandatory for acute gout studies: pain, joint swelling, joint tenderness, patient global, physician global, functional disability; and for chronic gout studies: serum urate, gout flares, tophus regression, health-related quality of life, functional disability, pain, patient global, physician global, work disability and joint inflammation. Several additional domains were considered discretionary.

Magnetic resonance imaging in the quantitative assessment of gouty tophi

Measurements of tophus size can be important in monitoring the course of gout therapy, as tophus resolution is proposed as one measure of success of treatment. This multicentre study assessed the intra- and interreader reproducibility of quantitative tophus volume measurements from magnetic resonance images (MRI) in subjects with palpable gouty tophi. Subjects first underwent radiographic imaging of a selected tophus followed by MRI before and at <or=5, 10 and 20-min after gadolinium administration. After choosing optimal parameters, subjects underwent pre- and postgadolinium-enhanced MRIs of a selected tophus on two occasions separated by 5-10 days. Unenhanced spin-echo images provided satisfactory tophi images and were less subject to interfering artefacts than gadolinium-enhanced gradient-echo images. Intrareader reproducibility was excellent, with no statistically significant difference in mean tophus volume between visits (mean difference - 0.05 +/- 0.97 cm3). A small but statistically significant difference in interreader mean tophus volume was detected (mean difference 0.89 +/- 2.05 cm3; p < 0.05). MRI can quantify tophus size in gout and deserves further comparison with other techniques for tophus size monitoring in assessing effects of gout therapy.

Febuxostat (TMX-67), a novel, non-purine, selective inhibitor of xanthine oxidase, is safe and decreases serum urate in healthy volunteers

In order to evaluate the safety, pharmacological properties, and urate-lowering efficacy of febuxostat, a non-purine, selective inhibitor of xanthine oxidase, a Phase 1, 2-week, multiple-dose, placebo-controlled, dose-escalation study was conducted in 154 healthy adults of both sexes. Daily febuxostat doses in the range 10 mg to 120 mg resulted in proportional mean serum urate reductions ranging from 25% to 70% and in proportional increases in maximum febuxostat plasma concentrations and area under plasma concentration versus time curves. Accompanying the hypouricemic effect were increases in serum xanthine concentrations, decreases in urinary uric acid excretion, and increases in urinary xanthine and hypoxanthine excretion, confirming inhibition of xanthine oxidase activity by febuxostat. Hepatic conjugation and oxidative metabolism were the major pathways of elimination of febuxostat from the body, and renal elimination did not appear to play a significant role. Although not uncommon, adverse events were mild and self-limited, and no deaths or serious adverse events were observed. Febuxostat is a safe and potent hypouricemic agent in healthy humans.

Attitudes towards mental health care in younger and older adults: similarities and differences

It is commonly assumed that older and younger adults have very different attitudes about seeking mental health services and that this is a major factor in reducing the use of mental health services by the elderly. However, little evidence exists to illustrate how elders actually perceive mental health care. Responses from a survey of 474 older adults age 65 and over were compared with data from a national survey of 1001 persons age 21-65. Results indicate similarities in many attitudes including likelihood of seeking treatment for severe mental disorders, importance of mental health care, and concerns about cost and coverage as barriers to care. Differences included use of services, perceptions about less severe disorders, referral sources, and preferred providers. Clinical, policy and public education implications are discussed.

Phosphoribosylpyrophosphate synthetase and the regulation of phosphoribosylpyrophosphate production in human cells

between purine nucleoside diphosphate inhibition and inorganic phosphate (Pi) activation; and intracellular concentration of the PRS1 isoform. The operation of additional determinants of rates of PRPP synthesis in human cells is suggested by: (1) multiple PRS isoforms with distinctive physical and kinetic properties; (2) nearly immediate activation of intracellular PRPP synthesis in response to mitogens, growth-promoters, and increased intracellular Mg2+ concentrations; (3) tissue-specific differences in PRS1 and PRS2 transcript and isoform expression; and (4) reversible association of PRS subunits with one another and/or with PRS-associated proteins (PAPs), as a result of which the catalytic and perhaps regulatory properties of PRS isoforms are modified.

Accelerated transcription of PRPS1 in X-linked overactivity of normal human phosphoribosylpyrophosphate synthetase

Phosphoribosylpyrophosphate (PRPP) synthetase (PRS) superactivity is an X-linked disorder characterized by gout with overproduction of purine nucleotides and uric acid. Study of the two X-linked PRS isoforms (PRS1 and PRS2) in cells from certain affected individuals has shown selectively increased concentrations of structurally normal PRS1 transcript and isoform, suggesting that this form of the disorder involves pretranslational dysregulation of PRPS1 expression and might be more appropriately termed overactivity of normal PRS. We applied Southern and Northern blot analyses and slot blotting of nuclear runoffs to delineate the process underlying aberrant PRPS1 expression in fibroblasts and lymphoblasts from patients with overactivity of normal PRS. Neither PRPS1 amplification nor altered stability or processing of PRS1 mRNA was identified, but PRPS1 transcription was increased relative to GAPDH (3- to 4-fold normal in fibroblasts; 1.9- to 2.4-fold in lymphoblasts) and PRPS2. Nearly coordinate relative increases in each process mediating transfer of genetic information from PRPS1 transcription to maximal PRS1 isoform expression in patient fibroblasts further supported the idea that accelerated PRPS1 transcription is the major aberration leading to PRS1 overexpression. In addition, modulated relative increases in PRS activities at suboptimal Pi concentration and in rates of PRPP and purine nucleotide synthesis in intact patient fibroblasts indicate that despite an intact allosteric mechanism of regulation of PRS activity, PRPS1 transcription is a major determinant of PRPP and purine synthesis. The genetic basis of disordered PRPS1 transcription remains unresolved; normal- and patient-derived PRPS1s share nucleotide sequence identity at least 850 base pairs 5' to the consensus transcription initiation site.

Spontaneous minute ventilation predicts readiness for extubation in mechanically ventilated preterm infants

OBJECTIVE

We designed an observational study to test the hypothesis that a comparison of two methods of minute ventilation, spontaneously generated with mechanically generated, would be a useful predictor of readiness for extubation in preterm infants, weighing <2000 gm, who require mechanical ventilation for >24 hours.

STUDY DESIGN

This observational study of 35 infants weighing < or = 2000 gm evaluated the comparison of spontaneously generated minute ventilation with mechanically generated minute ventilation to successfully predict readiness for extubation. After reaching entry criteria, infants were extubated if their spontaneously generated minute ventilation (while receiving endotracheal CPAP) was > or = 50% of the mechanically generated minute ventilation during assist/control ventilation.

RESULTS

Of the 35 infants who had a successful trial and were extubated, 30 (86%) remained extubated for at least 24 hours. Of the five infants who failed extubation, four developed apnea and one developed stridor. Thus, a spontaneous minute ventilation of > or = 50% of mechanically generated minute ventilation predicted readiness for extubation in 86% of the patients in this observation.

CONCLUSION

A spontaneously generated minute ventilation that is > or = 50% of the mechanically generated minute ventilation is an objective predictor of the readiness for extubation in low birth weight infants who have been weaned to modest ventilatory support.

Comparison of two methods of surfactant administration and the effect on dosing-associated hypoxemia

OBJECTIVE

The purpose of our study was to test the hypothesis that surfactant dosing through a proximal sideport adapter on an endotracheal tube leads to more dosing-associated hypoxemia compared with a method of dosing that uses a double-lumen endotracheal tube.

STUDY DESIGN

Using adequate sample size to compare significant changes in O2 saturation (power > 0.8, alpha < 0.05) we enrolled 36 infants with respiratory distress syndrome in this randomized trial. A 10% change in O2 saturation was considered clinically significant. Nineteen infants received 38 doses of surfactant through the sideport adapter. Seventeen infants received 31 doses of surfactant through the nonventilation lumen of a double-lumen endotracheal tube. Two main outcome measures were assessed: time-averaged O2 saturation values 30 minutes after dosing and the largest absolute fall in O2 saturation for each patient.

RESULTS AND CONCLUSIONS

Time-averaged O2 saturation measures were higher in the proximal sideport group (p = 0.02), but the magnitude of difference was probably not clinically significant. No significant difference was detected between groups when we compared largest absolute drop in O2 saturation. Secondary analyses found no effect of birth weight or dose number (second vs third dose) on either outcome measure.

Melatonin production in an aerobic photosynthetic bacterium: an evolutionarily early association with darkness

Melatonin was measured in a species of aerobic photosynthetic bacteria, Erythrobacter longus, grown in either constant light or constant dark. A radioimmunoassay was used to quantify melatonin levels and thin-layer chromatography to confirm the identity of melatonin immunoactivity. Melatonin levels were significantly higher (nearly 2.3-fold) in the dark-grown than in the light-grown samples. Also, the homogenates of the dark-grown bacteria retained melatonin-producing enzymatic activity, whereas the light-grown homogenates did not; melatonin levels extracted from the dark-grown homogenates increased with increasing extraction time, reaching as high as 29.2 ng.mg-1 protein at 120 min. Removal of membrane fragments from homogenates did not influence melatonin levels in light-grown homogenate, but this procedure increased melatonin levels in dark-grown homogenate, indicating that at least some of the enzymes in the pathway of melatonin production are not membrane-bound. This study is the second to demonstrate the presence of melatonin at the prokaryotic level, supporting the evidence that melatonin appeared very early in evolution. Its function in prokaryotes has not been determined, but may relate to its antioxidative actions.

Overexpression of the normal phosphoribosylpyrophosphate synthetase 1 isoform underlies catalytic superactivity of human phosphoribosylpyrophosphate synthetase

To define the enzymatic and genetic basis of X-linked phosphoribosylpyrophosphate synthetase (PRS) catalytic superactivity, we measured concentrations of X-linked PRS1 and PRS2 isoforms in cultured fibroblasts and lymphoblasts by immunoblotting after separation by polyacrylamide-urea isoelectric focusing. PRS1 comprised >80% of measurable PRS isoforms in all fibroblast strains, but PRS1 concentrations in cells from six affected males exceeded those in normal cells by 2-6-fold. PRS absolute specific activities (activity per mg of PRS isoforms) were comparable in all fibroblast strains and in purified recombinant normal PRS1, confirming selectively increased levels of PRS1 isoform as the enzymatic basis of PRS catalytic superactivity. Cloning, sequencing, and expression of normal subject- and patient-derived PRS cDNAs predicted normal translated region sequences for both PRS isoforms and revealed no differences in catalytic properties of recombinant PRS1. Normal and patient PRPS1 transcribed but untranslated DNA sequences were also identical. Northern blot analysis showed selective increase in relative concentrations of PRS1 transcripts in patient fibroblasts. In PRS catalytic superactivity, overexpression of the normal PRS1 isoform thus appears to result from an altered pretranslational mechanism of PRPS1 expression. In lymphoblasts, however, expression of this alteration is attenuated, explaining the absence of phenotypic expression of PRS catalytic superactivity in these cells.

The genetic and functional basis of purine nucleotide feedback-resistant phosphoribosylpyrophosphate synthetase superactivity

The genetic and functional basis of phosphoribosylpyrophosphate synthetase (PRS) superactivity associated with purine nucleotide inhibitor-resistance was studied in six families with this X chromosome-linked purine metabolic and neurodevelopmental disorder. Cloning and sequencing of PRS1 and PRS2 cDNAs, derived from fibroblast total RNA of affected male patients by reverse transcription and PCR amplification, demonstrated that each PRS1 cDNA contained a distinctive single base substitution predicting a corresponding amino acid substitution in the PRS1 isoform. Overall, the array of substitutions encompassed a substantial portion of the translated sequence of PRS1 cDNA. Plasmid-mediated expression of variant PRS1 cDNAs in Escherichia coli BL21 (DE3/pLysS) yielded recombinant mutant PRS1s, which, in each case, displayed a pattern and magnitude of purine nucleoside diphosphate inhibitor-resistance comparable to that found in cells of the respective patient. Kinetic analysis of recombinant mutant PRS1s showed that widely dispersed point mutations in the X chromosome-linked PRPS1 gene encoding the PRS1 isoform result in alteration of the allosteric mechanisms regulating both enzyme inhibition by purine nucleotides and activation by inorganic phosphate. The functional consequences of these mutations provide a tenable basis for the enhanced production of phosphoribosylpyrophosphate, purine nucleotides, and uric acid that are the biochemical hallmarks of PRS superactivity.

Inhibition of human 5-phosphoribosyl-1-pyrophosphate synthetase by 4-amino-8-(beta-D-ribofuranosylamino)-pyrimido[5,4-d]pyrimidine-5'- monophosphate: evidence for interaction at the ADP allosteric site

The kinetics of inhibition by the aminopyrimidopyrimidine nucleotide 4-amino-8-(beta-D-ribofuranosylamino)pyrimido[5,4-d]pyrimidine[-5' -monophosphate (APP-MP) were assessed with two human isozymes of 5-phosphoribosyl-1-pyrophosphate synthetase (PRS) (PRS1 and PRS2) and a mutant enzyme, S.M. PRS1, derived from an individual with PRS hyperactivity. In the presence of 1 mM potassium phosphate, APP-MP inhibited PRS1 and PRS2 with half-maximal inhibition (IC50) at 5.2 microM and 23.8 microM, respectively. The degree of inhibition for both enzymes was highly dependent on the phosphate concentration; IC50 values were 70 times higher in the presence of 50 mM potassium phosphate. APP-MP exhibited mixed noncompetitive-uncompetitive inhibition against PRS1, with a Kii value of 6.1 microM and a Kis value of 14.6 microM, and produced parabolic secondary plots of slope or intercept versus APP-MP concentration. In comparison, inhibition of PRS1 by ADP was of a mixed noncompetitive-competitive type, with a Kii value of 9.6 microM and a Kis value of 2.8 microM. A similar kinetic analysis was completed using S.M. PRS1, a mutant enzyme with a single amino acid substitution resulting in diminished sensitivity to feedback inhibition by nucleotides. The noncompetitive component of ADP inhibition of PRS1 was absent with S.M. PRS1 and ADP inhibition was purely competitive, with a Ki of 6.4 microM, APP-MP was a very poor inhibitor of S.M. PRS1, displaying uncompetitive characteristics and a Ki of 1.6 mM. These data indicate that APP-MP inhibits PRS1 with a strong element of noncompetitive inhibition and appears to interact specifically at the allosteric site used by ADP. These results contrast with those obtained with ADP, which has a strong component of ATP competitive inhibition and binds at the ATP site as well as at a second, allosteric, site.

Endogenous porphyrins in murine skin and transplanted PAM-212 squamous cell carcinoma tissues after injection of delta-aminolevulinic acid

After intraperitoneal (IP) injection of delta-aminolevulinic acid (ALA), the endogenous porphyrins in murine skin and tumor tissues were determined by a method involving solvent and acid extractions. The results showed that the total amount of porphyrins in the tumor tissues after ALA injection was much higher than that in the skin from the same mice, although the amount of porphyrins in the skin from the ALA-injected mice was higher than that from the saline-injected (control) mice. The porphyrins in the tumor were mostly protoporphyrin and coproporphyrin, with only a small amount of uroporphyrin. The optimum period for porphyrin accumulation in the tumor as well as in the skin was 1 hour after the injection of ALA. As the period was extended to 3 and 6 hours, the amount of porphyrins in these tissues decreased considerably. These findings could be valuable for further application of ALA in the photodynamic therapy of skin cancer.

Ocular timolol levels after drug withdrawal: an experimental model

Carbon 14-labelled timolol maleate was instilled into both eyes of 12 pigmented rabbits daily for 42 days. Drug levels in the aqueous humour and ocular tissues were measured up to 42 days after drug withdrawal. The results indicate that timolol concentrates mainly in melanotic tissues, with slow release. Even 42 days after withdrawal the drug was still present in pigmented ocular tissues. Timolol was detected in the aqueous up to 5 days after withdrawal. These findings explain the long-term depressant effect of topically administered timolol on aqueous production. We conclude that lower or less frequent doses of timolol should be considered in patients with glaucoma.

Flow-synchronized ventilation of preterm infants with respiratory distress syndrome

Asynchrony of delivered and spontaneous breaths in mechanically ventilated infants may impair gas exchange and prolong the need for assisted ventilation. We conducted a randomized, controlled trial of a patient-triggered, flow-synchronized ventilator on 30 preterm infants with respiratory distress syndrome who weighed between 1100 and 1500 gm at birth. Entry criteria included radiographic evidence of respiratory distress syndrome and the need for mechanical ventilation and surfactant replacement therapy. Patients were assigned to either conventional time-cycled, pressure-limited ventilation or patient-triggered, flow-synchronized ventilation in an assist/control mode. Otherwise clinical management was identical. Time to extubation was the primary outcome measure. Patients treated with flow-synchronized ventilation were weaned more rapidly and had a significantly shorter mean time to extubation than those treated with time-cycled, pressure-limited ventilation, 119 versus 271 hours, p = 0.0152. In addition, there was no difference in the rate of complications between the two groups. There were, however, considerable reductions in patient charges of $4344 per patient in the flow-synchronized ventilation group.

A simplified method for the determination of phosphoribosylpyrophosphate synthetase activity in hemolysates

Methods currently employed for measurement of phosphoribosylpyrophosphate synthetase (ribophosphate pyrophosphokinase; PRPPs) activity are cumbersome and expensive, requiring an auxiliary enzyme reaction, a radioactive nucleobase and multiple incubations, or do not allow a complete kinetic study. The aim of the present study is to describe a simplified, single step, non-isotopic method for determination of PRPPs in hemolysate, appropriate for a complete screening of PRPPs activity disorders. Briefly, the charcoal-treated hemolysate is incubated with saturating amounts of substrates and P1 P5 di(adenosine 5') pentaphosphate (Ap5A), an inhibitor of human adenylate kinase, to prevent conversion of AMP to ADP. AMP generated in this reaction is then measured by HPLC. Adenylate kinase activity was fully inhibited by Ap5A, allowing the accurate determination of AMP. The method was sensitive and reproducible and mean and variance values compared closely with those reported using other, more complicated, assay procedures. The hyperbolic curve relating Pi concentration to initial reaction velocity was shifted to sigmoidal by addition of 0.02 mmol/l GDP which inhibited PRPPs activities only at inorganic phosphate concentrations < 2 mmol/l. This suggests that this method should provide sensitive and accurate screening for regulatory, as well as catalytic, defects underlying PRPPs superactivity.

Human X-linked phosphoribosylpyrophosphate synthetase superactivity is associated with distinct point mutations in the PRPS1 gene

Superactivity of phosphoribosylpyrophosphate synthetase (PRS) is an X chromosome-linked disorder of purine metabolism, characterized by gout with uric acid overproduction and, in some families, neurodevelopmental impairment. Two highly homologous isoforms of PRS (PRS1 and PRS2), each encoded by a distinct X chromosome-linked locus, have been identified, and PRS1 and 2 cDNAs have been cloned. The entire 954-base pair translated regions of PRS1 and 2 cDNAs derived from cultured lymphoblasts and fibroblasts from two patients in whom purine nucleotide feedback resistance of PRS is associated with enzyme superactivity and neurodevelopmental defects were examined by direct sequencing after polymerase chain reaction amplification of PRS transcripts. Nucleotide sequences of PRS2 cDNAs from the patients and normal individuals were identical. In contrast, PRS1 cDNAs from the patients differ from normal PRS1 cDNA, each by a single base substitution. PRS1 cDNA from patient N. B. showed an A to G transition at nucleotide 341, corresponding to an asparagine to serine change at amino acid residue 113 of mature PRS1. A G to C transversion at nucleotide 547, indicating an aspartic acid to histidine change at amino acid 182, was found for PRS1 cDNA from patient S. M. Point mutations at the sites identified in the PRS1 cDNAs of the two patients were confirmed by the results of RNase mapping analysis. Normal, N. B., and S. M. PRS1 cDNAs were introduced into Escherichia coli BL21 (DE3)/pLyS, and recombinant N. B. and S. M. PRS1s showed the purine nucleotide feedback resistance phenotypes characteristic of PRS from patients' cells.

Overexpression, purification, and characterization of recombinant human 5-phosphoribosyl-1-pyrophosphate synthetase isozymes I and II

Human 5-phosphoribosyl-1-pyrophosphate synthetase isozymes I and II (PRSI and PRSII) have been isolated independently and characterized in pure form. cDNAs for PRSI and PRSII were overexpressed in an Escherichia coli strain which lacks the bacterial 5-phosphoribosyl-1-pyrophosphate synthetase. The recombinant isoforms were purified to virtual homogeneity with specific activities of 25.0 and 35.7 units/mg, respectively, values which are 5-10-fold higher than any previously reported for this enzyme from human sources. Despite 95% amino acid sequence identity, the isoforms differed significantly in several physical and kinetic properties. PRSII was more sensitive to heat inactivation at 55 degrees C and more susceptible to disaggregation to inactive forms in the absence of Mg2+ and ATP than was PRSI. The isoforms were separable on the basis of isoelectric point. PRSI and PRSII also differed significantly in Km values for MgATP and ribose 5-phosphate, pH optima, and Mg2+ and Pi activation curves. PRSII was less sensitive to feedback inhibition by purine nucleotides and more sensitive to inhibition by 2,3-diphosphoglycerate than PRSI. Differences in kinetic properties between PRSI and PRSII are consistent with the suggestion that PRSII predominates in rapidly proliferating cells.

Regulation of purine nucleotide synthesis in human B lymphoblasts with both hypoxanthine-guanine phosphoribosyltransferase deficiency and phosphoribosylpyrophosphate synthetase superactivity

Human B lymphoblast lines severely deficient in hypoxanthine-guanine phosphoribosyltransferase (HGPRT) were selected for resistance to 6-thioguanine from cloned normal and phosphoribosylpyrophosphate (PP-Rib-P) synthetase-superactive cell lines and were compared with their respective parental cell lines with regard to growth and PP-Rib-P and purine nucleotide metabolism. During blockade of purine synthesis de novo with 6-methylthioinosine or aminopterin, inhibition of growth of all HGPRT-deficient cell lines was refractory to addition of Ade at concentrations which restored substantial growth to parental cell lines. Ade-resistant inhibition of growth of parental lines by 6-methylthioinosine, however, occurred during Ado deaminase inhibition. Insufficient generation of IMP (and ultimately guanylates) to support growth of lymphoblasts lacking HGPRT activity and blocked in purine synthesis de novo best explained these findings, implying that a major route of interconversion of AMP to IMP involves the reaction sequence: AMP----Ado----Ino----Hyp----IMP. PP-Rib-P generation and purine nucleoside triphosphate pools were unchanged by introduction of HGPRT deficiency into normal lymphoblast lines, in agreement with the view that accelerated purine synthesis de novo in this deficiency results from increased availability of PP-Rib-P for the pathway. Cell lines with dual enzyme defects did not differ from PP-Rib-P synthetase-superactive parental lines in rates of PP-Rib-P and purine synthesis despite 5-6-fold increases in PP-Rib-P concentrations, excretion of nearly 50% of newly synthesized purines, and diminished GTP concentrations. Fixed rates of purine synthesis de novo in PP-Rib-P synthetase-superactive cells appeared to reflect saturation of the rate-limiting amidophosphoribosyltransferase reaction for PP-Rib-P. In combination with accelerated purine excretion, increased channeling of newly formed purines into adenylates, and impaired conversion of AMP to IMP, fixed rates of purine synthesis de novo may condition cell lines with defects in HGPRT and PP-Rib-P synthetase to depletion of GTP with consequent growth retardation.

Physical mapping of loci in the distal half of the short arm of the human X chromosome: implications for the spreading of X-chromosome inactivation

The relative order of 11 loci in the distal half of the short arm of the human X chromosome was examined using a panel of somatic cell hybrids containing structurally rearranged X chromosomes. The results show that the gene for phosphoribosylpyrophosphate synthetase 2 (PRPS2) is located between ZFX (zinc finger protein, X-linked) and STS (steroid sulfatase). The results also confirm the localization of ZFX distal to POLA (alpha-DNA polymerase). Previous studies have shown that STS and ZFX escape X-inactivation whereas POLA undergoes inactivation. Evaluation of PRPS2 expression in somatic cell hybrids containing inactive human X chromosomes showed that PRPS2 undergoes X-inactivation. These results provide further evidence for interspersion of loci that do and do not undergo X-inactivation on the human X chromosome.

Role of iron in the photosensitization by uroporphyrin

The role of iron in the mechanism of photosensitivity due to uroporphyrin was investigated. There is frequently increased levels of Fe in the serum from patients with porphyria cutanea tarda, where the photosensitivity is due to uroporphyrin. It has been reported that H2O2 has a major role in the uroporphyrin induced photosensitivity. Hence we examined the hypothesis that Fe would catalyze the production of OH from H2O2 and the OH thus formed may have a significant role in the uroporphyrin photosensitivity. This was examined by studying the effects of the Fe chelating compound deferoxamine in an in vitro system. Our results show that deferoxamine inhibited the uroporphyrin photosensitivity, but not the photosensitivity due to protoporphyrin. This indicates that Fe may play a role in the uroporphyrin photosensitization in the skin, by accelerating the formation of OH, which may be a major reactive species responsible for the photosensitization in porphyria cutanea tarda.

Cloning of cDNAs for human phosphoribosylpyrophosphate synthetases 1 and 2 and X chromosome localization of PRPS1 and PRPS2 genes

Cloned cDNAs representing the entire, homologous (80%) translated sequences of human phosphoribosylpyrophosphate synthetase (PRS) 1 and PRS 2 cDNAs were utilized as probes to localize the corresponding human PRPS1 and PRPS2 genes, previously reported to be X chromosome linked. PRPS1 and PRPS2 loci mapped to the intervals Xq22-q24 and Xp22.2-p22.3, respectively, using a combination of in situ chromosomal hybridization and human x rodent somatic cell panel genomic DNA hybridization analyses. A PRPS1-related gene or pseudogene (PRPS1L2) was also identified using in situ chromosomal hybridization at 9q33-q34. Human HPRT and PRPS1 loci are not closely linked. Despite marked cDNA and deduced amino acid sequence homology, human PRS 1 and PRS 2 isoforms are encoded by genes widely separated on the X chromosome.

Quantitation of hydrogen peroxide formed during UV-visible irradiation of protoporphyrin, coproporphyrin and uroporphyrin

Free porphyrins are strong photosensitizers. Previously reported findings indicate that the in vitro cell lysis induced by irradiation in the presence of coproporphyrin (CP) and uroporphyrin (UP) is mediated by H2O2 and that induced by irradiation with protoporphyrin (PP) is not mediated by H2O2. In the present study the possible role of H2O2 in the porphyrin photosensitization was investigated by direct measurement of the H2O2 formed during the irradiation of PP, CP and UP. Our results show that the amount of H2O2 formed decreased in the following order: UP, CP, PP. The amounts of H2O2 formed during irradiation of CP and PP were approximately 86% and 38% respectively in comparison to the H2O2 formed during the irradiation of UP. The formation of H2O2 was inhibited by sodium azide, a strong quencher of singlet oxygen. These observations are in good agreement with the previous report that the in vitro photolysis of Ehrlich ascites carcinoma cells by UP and CP, but not that by PP, was inhibited by catalase and clinical findings with patients with erythropoietic protoporphyria (EPP) and porphyria cutanea tarda (PCT). The patients with EPP, where the photosensitivity is due to PP, respond well to beta-carotene while beta-carotene does not protect against the photosensitivity in PCT, in which case the photosensitivity is due to uroporphyrin.

Inherited superactivity of phosphoribosylpyrophosphate synthetase: association of uric acid overproduction and sensorineural deafness

PURPOSE

Superactivity of 5-phosphoribosyl 1-pyrophosphate (PP-Rib-P) synthetase, inherited as an X chromosome-linked trait, has been reported in nearly 20 families in which overproduction of uric acid is invariably present in hemizygous affected males. Clinical manifestations of PP-Rib-P synthetase superactivity are mainly limited to gout in early adulthood. Neurologic deficits, including sensorineural deafness, have rarely been described. We herein document the association of PP-Rib-P synthetase superactivity, gout with excessive uric acid synthesis, and sensorineural deafness in an additional family.

PATIENTS AND METHODS

Two members of a Spanish family were studied: an eight-year-old boy (Patient 1) with tophaceous gout, purine nucleotide and uric acid overproduction, and sensorineural deafness, and his 27-year-old mother (Patient 2), who had gout. Fibroblast cultures were initiated from skin biopsy specimens, and measurements of PP-Rib-P and purine nucleotide metabolism in the fibroblasts were performed.

RESULTS

A labile but superactive PP-Rib-P synthetase was demonstrated in the fibroblasts cultured from both Patients 1 and 2. The kinetic basis of PP-Rib-P synthetase superactivity in this family was resistance to purine nucleotide inhibition of enzyme activity. More severe derangements in the enzyme and in PP-Rib-P and purine synthesis in Patient 1's cells than in Patient 2's cells suggest that Patient 1 is hemizygous and Patient 2 is heterozygous for an X chromosome-linked genetic defect. Limited pedigree data support this view. Compared with affected members of seven other families with PP-Rib-P synthetase superactivity, these patients are intermediate in the range of clinical expression and in the severity of the enzyme defect as measured by the degree of aberration of PP-Rib-P and purine nucleotide synthesis in fibroblasts. Metabolic abnormalities were more severe in Patient 1's cells than in the cells of most male patients (in whom clinical expression is limited to early adult-onset gout) but were less severe than in the cells of two patients in whom more complex enzyme defects were associated with uric acid overproduction and neurodevelopmental abnormalities (including deafness) in male children and adult women.

CONCLUSION

Certain defects resulting in PP-Rib-P synthetase superactivity may be causally related to neurologic impairment, most commonly sensorineural deafness.

Clinical aspects of monosodium urate monohydrate crystal deposition disease (gout)

Gout is a clinical syndrome with a limited range of manifestations arising as a result of the deposition of crystals of monosodium urate, the final product of purine metabolism in humans. Hyperuricemia is a common chemical aberration that is most often mild and remains asymptomatic. Thus, hyperuricemia should be distinguished from gout, even though urate supersaturation is necessary for the expression of gout. Uric acid overproduction and diminished renal uric acid excretion are the major mechanisms resulting in hyperuricemia, and an understanding of the basis of hyperuricemia in individual gout patients is an important step in determining appropriate treatment and in identifying underlying disorders, offending drugs and toxins, and inherited enzyme defects, all of which can result in hyperuricemia and gout. A scheme is presented for the evaluation of patients with new-onset gout, along with a discussion of the relationships between gout/hyperuricemia and a variety of metabolic disorders that are unusually prevalent in gouty populations.

Regulation of purine synthesis de novo in human fibroblasts by purine nucleotides and phosphoribosylpyrophosphate

Previous studies of purine nucleotide synthesis de novo have suggested that major regulation of the rate of the pathway is affected at either the phosphoribosylpyrophosphate (PP-Rib-P) synthetase reaction or the amidophosphoribosyltransferase (amido PRT) reaction, or both. We studied control of purine synthesis de novo in cultured normal, hypoxanthine-guanine phosphoribosyltransferase (HGPRT)-deficient, and PP-Rib-P synthetase-superactive human fibroblasts by measuring concentrations and rates of synthesis of PP-Rib-P and purine nucleotide end products, proposed effectors of regulation, during inhibition of the pathway. Incubation of cells for 90 min with 0.1 mM azaserine, a glutamine antagonist which specifically blocked the pathway at the level of conversion of formylglycinamide ribotide, resulted in a 5-16% decrease in purine nucleoside triphosphate concentrations but no consistent alteration in generation of PP-Rib-P. During this treatment, however, rates of the early steps of the pathway were increased slightly (9-15%) in normal and HGPRT-deficient strains, more markedly (32-60%) in cells with catalytically superactive PP-Rib-P synthetases, and not at all in fibroblasts with purine nucleotide feedback-resistant PP-Rib-P synthetases. In contrast, glutamine deprivation, which inhibited the pathway at the amido PRT reaction, resulted in time-dependent nucleoside triphosphate pool depletion (26-43% decrease at 24 h) accompanied by increased rates of PP-Rib-P generation and, upon readdition of glutamine, substantial increments in rates of purine synthesis de novo. Enhanced PP-Rib-P generation during glutamine deprivation was greatest in cells with regulatory defects in PP-Rib-P synthetase (2-fold), but purine synthesis in these cells was stimulated only 1.4-fold control rates by glutamine readdition. Stimulation of these processes in normal and HGPRT-deficient cells and in cells with PP-Rib-P synthetase catalytic defects was, respectively: 1.5 and 2.0-fold; 1.5 and 1.7-fold; and 1.6 and 4.1-fold. These studies support the following concepts. 1) Rates of purine synthesis de novo are regulated at both the PP-Rib-P synthetase and amido PRT reactions by end products, with the latter reaction more sensitive to small changes in purine nucleotide inhibitor concentrations. 2) PP-Rib-P exerts its role as a major regulator of purine synthetic rate by virtue of its interaction with nucleotide inhibitors to determine the activity of amido PRT. 3) Activation of amido PRT by PP-Rib-P is nearly maximal at base line in fibroblasts with regulatory defects in PP-Rib-P synthetase.