Homocysteine as a risk factor for nephropathy and retinopathy in Type 2 diabetes

AIMS/HYPOTHESIS

The aim of this study was to examine the relation between serum total homocysteine concentrations and microvascular complications in Pima Indians with Type 2 diabetes.

METHODS

Homocysteine concentrations were measured in frozen sera of 396 diabetic participants in a longitudinal study who were 40 years of age or older and who had attended one or more examinations between 1982 and 1985. Retinopathy was assessed by fundoscopy and nephropathy by an albumin:creatinine ratio greater than 300 mg/g. The incidence rate ratio for a 5 micro mol/l difference in homocysteine was calculated using proportional hazard regression.

RESULTS

The incidence of each complication was assessed in subjects without that complication at baseline and with more than one follow-up examination: 229 for nephropathy, 212 for retinopathy and 266 for proliferative retinopathy. There were 101 incident cases of nephropathy, 113 of retinopathy and 40 of proliferative retinopathy during a mean follow-up of 8.6, 7.5 and 8.9 years, respectively. Incidence of nephropathy was associated with homocysteine concentrations: IRR=1.42 (95% CI, 1.09-1.84, p=0.01); this remained statistically significant controlled for age, sex and duration of diabetes (p=0.03), but not when controlled for baseline renal function (p=0.4). Homocysteine concentrations were not associated with the incidence of any retinopathy IRR=1.14 (95%CI 0.89-1.46, p=0.3) but were associated with the incidence of proliferative retinopathy IRR=1.62 (95% CI 1.16-2.28, p=0.005); this association remained statistically significant when controlled for baseline renal function and diabetes duration (p=0.02).

CONCLUSIONS/INTERPRETATION

Increased homocysteine concentrations are associated with an increased risk for incidence of nephropathy and proliferative retinopathy; the relation with incidence of nephropathy seems to be explained by an association with baseline albuminuria status concentrations, whereas the relation with incidence of proliferative retinopathy does not.

Determination of 5-methyltetrahydrofolic acid in human serum by stable-isotope dilution high-performance liquid chromatography-mass spectrometry

The need for specific and sensitive methods for the determination of distinct serum folates is of high priority in clinical research settings. A stable-isotope liquid chromatography-mass spectrometry (LC/ESI-MS) assay was developed for the quantitative determination of the monoglutamyl form of 5-methyltetrahydrofolic acid (5-MTHFA) in human serum. Serum samples (0.5 ml) were amended with the internal standard, [5-13C5]MTHFA that had been labeled on the glutamic acid portion of the molecule and allowed to equilibrate. The analyte was trapped onto a solid-phase cartridge and then eluted with the HPLC mobile phase. Forty microliters was taken for LC/ESI-MS analysis using electrospray ionization operated in the positive ion mode. Using the standard method of addition of 5-MTHFA to serum, a linear dilution curve (y = 12.777x - 1.404; range 0.94-97 ng x ml(-1)) was constructed. The precision of the method was 5.3% (CV) based on the analysis of four sample replicates. The mass spectrum produced upon collision induced dissociation of the analyte in serum was used to confirm the identity of the 5-MTHFA. The method was applied to the analysis of a set of serum samples that contained standardized concentrations of 5-MTHFA. The determinations of 5-MTHFA in these samples using the LC/ESI-MS procedure were found to be in good agreement with other folate methods. A highly accurate and specific method for the analysis of 5-MTHFA in serum has been developed utilizing stable isotope dilution mass spectrometry.

Analysis of factors influencing the comparison of homocysteine values between the Third National Health and Nutrition Examination Survey (NHANES) and NHANES 1999+

Two important changes occurred in the time between the Third National Health and Nutrition Examination Survey (NHANES III) (1991-1994) and the later survey (NHANES 1999+) regarding total homocysteine (tHcy), i.e., a change in matrix from serum to plasma and a change in analytical methods. The goals of this study were to determine the magnitude of potential differences between plasma and serum with regard to tHcy concentrations, and between the two analytical methods used in these surveys. Optimally prepared plasma, serum allowed to clot for 30 and 60 min at room temperature and serum allowed to clot for 30 and 60 min and subjected to four freeze-thaw cycles, prepared from blood samples collected from 30 healthy people, were analyzed by both methods. Serum samples had significantly higher tHcy concentrations than plasma samples, and the difference increased with longer clotting time. Freeze-thaw cycles had little or no effect on the variability or bias in the serum sample results. The tHcy results produced by the two analytical methods were significantly different, but consistent across sample types. On average, the results of the method used in NHANES III were lower by 0.64 micromol/L; however, the relative bias varied with tHcy concentration. The tHcy results determined in surplus serum from NHANES III overestimated tHcy concentrations by approximately 10% compared with optimally prepared plasma. The average method bias was 6% between the two analytical methods. On the basis of changes in matrix and methodology, direct comparison of tHcy results between the two surveys is inappropriate.

Comparison of serum and plasma methylmalonic acid measurements in 13 laboratories: An international study

BACKGROUND

Detection of cobalamin deficiency is increasingly important, and methylmalonic acid (MMA) appears to be a useful marker. Information on interlaboratory variation and on methodological differences for MMA in serum and plasma is limited.

METHODS

Using gas chromatography/mass spectrometry, 13 laboratories participated in a 2-day analysis of 8 serum and 11 EDTA-plasma specimens. Results were analyzed for imprecision, recovery, and differences among laboratories and methods.

RESULTS

The mean among-laboratory imprecision (CV) was 19% and 21% for serum and plasma samples, respectively, and 9.3% and 7.8% for serum and plasma samples with added MMA, respectively. The mean within-laboratory (among-run) CV was 13% for both serum and plasma samples and 5.2% and 4.9% for serum and plasma samples with added MMA. Within-method imprecision was the same or higher than among-method imprecision. The mean among-laboratory recovery of MMA was 105% and 95% in serum and plasma, respectively. Most laboratories showed a proportional bias relative to the consensus mean of up to 15%. Two laboratories reported results that on average were almost 30% higher than the consensus mean.

CONCLUSIONS

No method differences were found, but significant among-laboratory imprecision was found in the present study. Improvements are needed to reduce the analytical imprecision of most laboratories, and attention must be focused on calibration issues. Differences among laboratories can be improved by introducing high-quality reference materials and by instituting external quality assessment programs.

Comparison of plasma total homocysteine measurements in 14 laboratories: an international study

BACKGROUND

Information on interlaboratory variation and especially on methodological differences for plasma total homocysteine is lacking.

METHODS

We studied 14 laboratories that used eight different method types: HPLC with electrochemical detection (HPLC-ED); HPLC with fluorescence detection (HPLC-FD) further subdivided by type of reducing/derivatizing agent; gas chromatography/mass spectrometry (GC/MS); enzyme immunoassay (EIA); and fluorescence polarization immunoassay (FPIA). Three of these laboratories used two methods. The laboratories participated in a 2-day analysis of 46 plasma samples, 4 additional plasma samples with added homocystine, and 3 plasma quality-control (QC) pools. Results were analyzed for imprecision, recovery, and methodological differences.

RESULTS

The mean among-laboratory and among-run within-laboratory imprecision (CV) was 9.3% and 5.6% for plasma samples, 8.8% and 4.9% for samples with added homocystine, and 7.6% and 4.2% for the QC pools, respectively. Difference plots showed values systematically higher than GC/MS for HPLC-ED, HPLC-FD using sodium borohydride/monobromobimane (however, for only one laboratory), and EIA, and lower values for HPLC-FD using trialkylphosphine/4-(aminosulfonyl)-7-fluoro-2,1,3-benzoxadiazole. The two HPLC-FD methods using tris(2-carboxyethyl) phosphine/ammonium 7-fluoro-2,1,3-benzoxadiazole-4-sulfonate (SBD-F) or tributyl phosphine/SBD-F, and the FPIA method showed no detectable systematic difference from GC/MS.

CONCLUSIONS

Among-laboratory variations within one method can exceed among-method variations. Some of the methods tested could be used interchangeably, but there is an urgent need to improve analytical imprecision and to decrease differences among methods.

Absorption of folate from fortified cereal-grain products and of supplemental folate consumed with or without food determined by using a dual-label stable-isotope protocol

The absorption of folic acid in fortified white and whole-wheat bread, rice, or pasta or in solution was evaluated in human subjects with use of a single-dose, dual-label, stable-isotope protocol that did not involve prior loading of subjects with nonlabeled folate. In each of five sequential trials, 14 adults received a single oral dose of [13C5]folic acid in one of the four fortified cereal-grain products or in water concurrently with an intravenous injection of [2H2]folic acid. In two additional trials, subjects received oral [13C5]folic acid with or without a light breakfast meal. In all trials, urine was collected 24-36 h postdosing and the isotopic labeling of urinary folates determined. Isotope excretion ratios of urinary folates (% [13C5]folate dose/% [2H2]folate dose), which were used as criteria of absorption, indicated no significant differences among the various fortified foods and the control (P = 0.607). Because statistical power was sufficient to have detected a 50% difference from the control, these results suggest that [13C5]folic acid in these fortified cereal-grain foods was highly available. This study also suggests that fortification will contribute effectively to the folate status of the population. Consuming [13C5]folic acid after a light breakfast meal led to a small reduction in absorption relative to the control without food (P < 0.085). Between-subject variation in this protocol exceeded that observed in previous studies conducted using prior saturation of subjects with nonlabeled folic acid. We recommend that either prior saturation or multiple doses be used in future applications of this technique to improve precision.

A dual-label stable-isotopic protocol is suitable for determination of folate bioavailability in humans: evaluation of urinary excretion and plasma folate kinetics of intravenous and oral doses of [13C5] and [2H2]folic acid

Stable isotopic protocols for the study of folate absorption were conducted to determine the following: (1) the equivalence of the [13C5] and [2H2] forms of folic acid, and (2) the merits of short-term plasma kinetics from injected and oral doses vs. urinary excretion of [13C5] and [2H2]folates. Another objective was to evaluate the merits of protocols not involving "saturation" of subjects with nonlabeled folate. Oral administration of [13C5] and [2H2]folic acid ( approximately 500 nmol each) to adult subjects (n = 4) yielded an equivalent 24-h urinary excretion of approximately 2% of each dose (molar ratio of urinary [13C5]/[2H2]folates = 0.96 +/- 0.055; mean +/- SEM). Expression of urinary excretion as a ratio of [13C5]/[2H2]folates yielded less within-group variability than seen for absolute excretion of each form of labeled folate. In the second study, subjects received 226 nmol of [2H2]folic acid intravenously and 1010 nmol of [13C5]folic acid orally. Isotopic enrichment of plasma [2H2]folates rose rapidly and returned to near basal values by approximately 2 h postdose. In contrast, enrichment of plasma [13C5]folates was detected until 4 h after dose, whereas enrichment values were far lower than seen with [2H2]folate. Adjusting for the difference in dose, the molar response of plasma area under the curve for isotopic enrichment was 15- to 20-fold greater for injected folates. In view of this very limited short-term plasma response even with a relatively large oral dose, presumably due to hepatic first-pass uptake, these findings suggest that plasma kinetics would be of limited usefulness in assessing the relative bioavailability of nutritionally relevant oral doses of labeled folate.

Enzymatic deconjugation of erythrocyte polyglutamyl folates during preparation for folate assay: investigation with reversed-phase liquid chromatography

Erythrocyte (RBC) folates occur mainly as 5-methyltetrahydrofolate polyglutamates. Determination of RBC folate concentration requires an initial deconjugation of these polyglutamates. In this study, existing HPLC methods were adapted to investigate the rate and extent of this deconjugation process. The action of endogenous plasma pteroyl-polyglutamate hydrolase activity was strongly affected by the conditions of sample preparation, with pH of the incubation mixture more critical to effective deconjugation than incubation time. Dilution of whole blood with 10 g/L ascorbic acid yielded fast hydrolysis of long-chain polyglutamates, and total conversion to 5-methyltetrahydrofolate monoglutamate occurred after 90 min of incubation at 37 degrees C. In contrast, dilution of whole blood with 10 g/L sodium ascorbate, with up to 90 min of incubation at 37 degrees C, yielded a mixture of polyglutamates of 5-methyltetrahydrofolate (glun = 1-8). As documented by direct HPLC analysis and in concurrent assays with Lactobacillus casei, acidification provided by ascorbic acid can have dramatic effects on the measurement of RBC folates.

Enzymatic deconjugation of erythrocyte polyglutamyl folates during preparation for folate assay: investigation with reversed-phase liquid chromatography

Erythrocyte (RBC) folates occur mainly as 5-methyltetrahydrofolate polyglutamates. Determination of RBC folate concentration requires an initial deconjugation of these polyglutamates. In this study, existing HPLC methods were adapted to investigate the rate and extent of this deconjugation process. The action of endogenous plasma pteroyl-polyglutamate hydrolase activity was strongly affected by the conditions of sample preparation, with pH of the incubation mixture more critical to effective deconjugation than incubation time. Dilution of whole blood with 10 g/L ascorbic acid yielded fast hydrolysis of long-chain polyglutamates, and total conversion to 5-methyltetrahydrofolate monoglutamate occurred after 90 min of incubation at 37 degrees C. In contrast, dilution of whole blood with 10 g/L sodium ascorbate, with up to 90 min of incubation at 37 degrees C, yielded a mixture of polyglutamates of 5-methyltetrahydrofolate (glun = 1-8). As documented by direct HPLC analysis and in concurrent assays with Lactobacillus casei, acidification provided by ascorbic acid can have dramatic effects on the measurement of RBC folates.

Osteotomy of the metacarpals and phalanges stabilized by AO plates and screws

Although digital or metacarpal nonunion with resultant functional disability of the hand is encountered often enough that corrective osteotomy becomes a consideration, many surgeons are reluctant to attempt osteotomy in the hand. This reluctance is reflected in the sparse number of cases reported in the literature. Since the effectiveness of AO plates and screw fixation for acute phalangeal and metacarpal fractures is well established, it seems logical to perform osteotomies in phalanges or metacarpals and stabilize them in the same fashion. We have reviewed 36 such osteotomies from the files of the AO Documentation Center. The osteotomies were done through either phalanges or metacarpals to correct angulation and/or rotary malunion, and yielded twenty-three very good, eight good and five poor results, or an overall satisfactory result rate of 86%. All osteotomy sites healed and the poor results reflect persistent deformity or limited adjacent joint motion. Thus, it appears that previously expressed pessimism regarding potential non-union or joint stiffness following phalangeal or metacarpal osteotomy is not warranted in angular and/or rotational osteotomy in the hand can yield satisfactory results in a very high percentage of patients.