Serum N-Acetyl-β-d-Glucosaminidase Activity in Predicting the Development of Hypertension
Abstract We conducted a prospective study in residents of a small farming community in southwestern Japan to determine whether elevated serum N-acetyl-β-d-glucosaminidase (NAG) activity would predict future hypertension. The 505 normotensive subjects (blood pressure, <140/90 mm Hg; mean age, 52±12 years) were reexamined after 7 years; 111 (22%) had become hypertensive (defined as blood pressure ≥140/90 mm Hg and/or taking antihypertensive medication at follow-up). After adjustment for age and sex, the development of hypertension was significantly related to body mass index (P<.002), the sum of skinfolds (P<.001), baseline blood pressure (P<.0001), serum cholesterol (P<.01), serum uric acid level (P<.0001), and serum NAG activity (P<.005). Elevated NAG activity showed an independent relationship to future hypertension (P<.005) after adjustments for age, sex, baseline blood pressure (systolic, diastolic, or mean), uric acid level, and the sum of skinfolds. Therefore, elevated serum NAG activity was an effective indicator of future hypertension, and it might therefore be related to functional and/or structural changes in the cardiovascular system.
N-Acetyl-β-d-glucosaminidase (NAG) is a lysosomal enzyme widely distributed in human tissue. It is released into serum from cells by exocytosis or from the breakdown of cells.1 Because of its high molecular weight (130 000 to 140 000 D), NAG is not filtered through the glomerular membrane. In the presence of a glomerular lesion or tubular damage, urinary NAG activity increases and therefore has an important diagnostic application in renal diseases.2 Urinary NAG is reportedly elevated in cases of essential hypertension with target-organ damage.3 In untreated mild hypertension without detectable organ damage, data have been conflicting.4 5 On the other hand, serum NAG is reportedly increased in complicated and uncomplicated essential hypertension.4 6 Schmieder et al4 reported that serum NAG activity was greater in patients with essential hypertension than in normotensive subjects, and there was a positive correlation between diastolic pressure and serum NAG activity in essential hypertension. Furthermore, hypertensive patients with high serum NAG activity showed a greater increase in systolic pressure and a greater total peripheral resistance in response to the cold pressor test as well as a greater increase in blood pressure (BP) in response to bicycle exercise testing than a group with low serum NAG activity. Schmieder et al suggested that serum NAG could be an important diagnostic marker that reflects the functional and/or structural cardiovascular changes in the early stages of essential hypertension.
We previously reported that elevated serum NAG activity was related to various cardiovascular risk factors, including high BP, in the general population.7 This cross-sectional association between BP and serum NAG activity, along with other clinical reports of high serum NAG activity in hypertension,4 5 6 formed our basis to design and carry out a prospective study on the longitudinal association between serum NAG activity and BP in normotensive subjects.
During a periodic epidemiological survey performed in 1982 in a small farming community in southwestern Japan (Tanushimaru town, Fukuoka Prefecture), we studied 77.3% of the population aged 20 years or older.7 In 1989, we conducted a follow-up examination in 779 subjects aged 33 years or older at the time of the previous survey; the follow-up rate was 82%. The 505 subjects who were at that time untreated and normotensive (mean age, 52±12 years) were used for the present study. Their BP in that baseline survey and also in the recent follow-up was classified according to World Health Organization (WHO) criteria.8 Normotension was defined as a BP less than 140/90 mm Hg. Subjects with borderline hypertension (140-159/90-94 mm Hg) or hypertension (≥160/95 mm Hg or taking medication) at follow-up were regarded as having developed hypertension.
The protocol for the entry and follow-up examinations was similar to the one used in the Seven Countries study.9 Briefly, a dietitian conducted a dietary survey, and the subjects’ medical history and use of alcohol and cigarettes were ascertained by questionnaire. Height and weight were measured, and body mass index (kilograms per meter squared) was calculated as an index of obesity. Skinfold thickness was measured in the triceps muscle of the arm and in the subscapular area with the use of a subcutaneous fat caliper (Eiyoken, Meiko Co); the sum of these skinfold measurements was calculated.
BP was measured three times with subjects in the supine position by a team of physicians. The third measurement with the fifth phase diastolic pressure was used for analysis. Mean arterial BP was calculated as diastolic pressure plus one third pulse pressure.
Blood was drawn from the antecubital vein for determination of lipids, serum creatinine, total protein, serum albumin, uric acid, and NAG. Serum NAG was measured by a colorimetric method with the use of an artificial substrate (NAG Test Shionogi).10 Other chemistries were measured by an autoanalyzer (ACA 8000, Olympus).
Measurements were performed in the 1989 follow-up survey in the same fashion as in the baseline survey except for NAG measurement. This study was approved by the Japan Medical Association of Ukiha (Tanushimaru) branch and by the local citizens’ committee of Tanushimaru. All participants gave informed consent.
Results are presented as mean±SD. Mean differences were tested by ANOVA. The χ2 test was used for evaluation of categorical parameters. The Mantel-Haenszel χ2 test was used for testing of the statistical significance of high NAG activity for the development of hypertension, with the difference in systolic pressure levels (≥120 versus <120 mm Hg) taken into consideration.
Multiple logistic regression analysis was performed with age and sex as covariates. Adjustments for age, sex, BP, sum of skinfolds, and uric acid were incorporated for investigation of the independent relationship between serum NAG activity and the development of hypertension. Sex, alcohol consumption, current smoking, proteinuria, and glycosuria were used as dummy variables. A level of P<.05 was accepted as statistically significant.
Background of Subjects
Table 1⇓ shows the characteristics of the subjects stratified by the WHO classification of BP at follow-up. Borderline hypertension was observed at follow-up in 59 subjects and hypertension in 52 subjects (52% of these 52 subjects were taking antihypertensive medication).
The subjects who developed borderline hypertension or hypertension were older and had higher mean body mass index, higher baseline BP levels, lower serum albumin concentration, higher cholesterol levels, and higher uric acid concentration in the study entry than those who did not develop hypertension. Serum NAG activity was also higher in the subjects who developed borderline hypertension or hypertension. Serum NAG activity in subjects taking antihypertensive medication was not different (14.1±3.6 IU/L) from that in borderline hypertensive subjects (13.4±3.5 IU/L) or hypertensive subjects not taking medication (12.4±3.1 IU/L). Total protein, high-density lipoprotein cholesterol, and serum creatinine levels showed no significant mean difference among the three groups. Similar percentages of subjects in the three groups reported alcohol consumption (20% to 24%) and cigarette use (21% to 25%).
The relationships between various parameters in the baseline study and the development of hypertension were evaluated after adjustment for age and sex (Table 2⇓). The sum of skinfolds; body mass index; systolic, mean, and diastolic BP values; serum cholesterol; serum NAG activity; and serum uric acid level were significantly related to the development of hypertension. Among these parameters, baseline systolic pressure had the best correlation with future hypertension. After further adjustment for systolic pressure, further significance was observed for the sum of skinfolds (P<.004), body mass index (P<.03), serum uric acid levels (P<.002), and serum NAG activity (P<.005). Further adjustment for uric acid resulted in a great decrease in the predictive power of the sum of skinfolds (P<.02) and of body mass index (P=.16, NS), indicating some effect between the indexes of obesity and uric acid concentration, whereas serum NAG activity (P<.005) was not affected. After adjustment for serum NAG, the sum of skinfolds lost its significance (P=.07, NS).
Baseline systolic pressure had the best correlation among several factors with future hypertension. Fig 1⇓ shows the relationships among baseline systolic pressure, serum NAG activity, and future hypertension. Systolic pressure was arbitrarily divided into two groups: greater than or equal to 120 mm Hg and less than 120 mm Hg. Serum NAG activity was also arbitrarily divided into two groups: greater than or equal to 12 IU/L and less than 12 IU/L. Two (1%) of 141 people who had lower systolic pressure and lower NAG (A in Fig 1⇓), 21 (18%) of 118 people who had lower systolic pressure and higher NAG (B in Fig 1⇓), 39 (34%) of 116 people who had higher systolic pressure and lower NAG (C in Fig 1⇓), and 49 (38%) of 130 people who had higher systolic pressure and higher NAG (D in Fig 1⇓) developed hypertension at follow-up (P<.01). The relative risk (95% confidence interval) of high serum NAG activity greater than or equal to 12 IU/L for the development of hypertension was 2.0 (1.2-3.2), with the difference in systolic pressure levels taken into consideration.
Fig 2⇓ shows the predicted risk for the development of hypertension (≥140/90 mm Hg or use of medication) in men aged 50 years with systolic pressure of 130 mm Hg and uric acid concentration of 252 mmol/L (4.4 mg/dL) with increasing serum NAG activity.
Serum NAG Activity as Predictor of Future Hypertension
We assessed the relationship between serum NAG activity and future hypertension by adjusting for various parameters (Table 3⇓). A decrease in the predictive power of serum NAG was demonstrated after adjustments for age and sex and further adjustments for the sum of skinfolds, indicating the interaction of these parameters with serum NAG activity in predicting hypertension. However, serum NAG activity remained independently related to the development of hypertension. Adjustments for diastolic pressure or mean BP instead of systolic pressure did not influence the predictive power of serum NAG activity. This association was not influenced by a further adjustment for serum creatinine, a trace or higher positive proteinuria, or a trace or higher positive glycosuria (P<.002), although the number of subjects decreased from 505 to 486 because of missing urinalysis data. The same analyses used in Table 3⇓ were repeated in 393 subjects, excluding the subjects with a trace or higher positive proteinuria (n=82), a trace positive glycosuria (n=12), or abnormal serum creatinine values (n=3). No change was observed in the association between serum NAG activity and the development of hypertension (P<.002).
Serum NAG activity has recently gained increasing importance as an aid in the diagnosis of hypertension, diabetes, atherosclerosis, and renal diseases, especially for the cardiovascular status in these diseases. Elevated serum NAG activity previously has been reported to be associated with various cardiovascular risk factors (age, high BP, elevated serum cholesterol, obesity, increased uric acid, and low high-density lipoprotein cholesterol) in the general population.7 This suggested that serum NAG activity might be a useful index for cardiovascular impairment and cardiovascular risk factors. The present study showed a significant correlation between serum NAG activity and future arterial hypertension in a population that was normotensive on entry. This correlation was independent of many variables, including age, sex, BP (systolic, mean, or diastolic), the sum of skinfolds, uric acid concentration, and body mass index. However, the mechanism of this correlation is uncertain.
Belfiore et al11 reported that serum NAG activity in atherosclerotic patients with angina pectoris, myocardial infarction, or cerebral vascular accidents within 6 months before study was 30% higher than that in healthy control subjects. In their study, enzymes such as aspartate aminotransferase or lactate hydrogenase, which are mainly or exclusively located in the cytosol, were not elevated, suggesting that the serum NAG elevation was not caused by a breakdown of cells but by the extracellular secretion of this lysosomal enzyme (exocytosis). Schmieder et al4 reported elevated serum NAG activity in subjects with untreated essential hypertension without detectable cardiovascular complications; this enzyme elevation returned to normal with the use of antihypertensive medication, suggesting that elevated serum NAG activity was related to early adaptive cardiovascular changes in subjects with essential hypertension. The report of Schmieder et al was interesting in that it suggested that elevated serum NAG activity was a consequence of hypertension, not a cause. On the contrary, Simon and Altman6 reported that pharmacological treatment of essential hypertension did not reverse the enzyme elevation, suggesting that arterial hypertension itself was not the cause of elevated serum NAG activity.
We have studied subjects who were initially normotensive. Therefore, elevated NAG activity initially would have to have originated from a cause or causes other than hypertension. Among the cardiovascular risk factors related to high serum NAG level, age, obesity,12 and uric acid13 have been found to be related to the development of hypertension. Adjustment for these factors demonstrated a slight decrease in the predictive power of serum NAG activity, indicating a mild interaction between serum NAG and the known risk factors for the development of hypertension. However, elevated serum NAG activity was still independently associated with future hypertension (Table 3⇑). The fact that young (20 to 39 years old) and thin (body mass index <25 kg/m2) individuals with borderline or established hypertension showed higher serum NAG activity than normotensive subjects in a cross-sectional analysis at entry (unpublished observations) suggested that serum NAG activity might be a marker for increased BP at least independent of age and obesity. If the elevation of serum NAG activity in patients with essential hypertension was connected to general nonspecific structural and/or functional changes in the cardiovascular system,4 it would be logical to infer that elevated serum NAG activity resulted from a pathological process in the cardiovascular system caused by internal factors (including genetic ones) and/or environmental factors.
Increased echographic left ventricular mass in normotensive subjects has been reported to be an independent predictor of arterial hypertension.14 Therefore, left ventricular hypertrophy may be caused by factors other than hypertension in these cases. The renin-angiotensin system, growth factors, genetic predisposition, or other unknown factors besides high BP could play roles in the increased left ventricular mass in essential hypertension. These factors affect not only the heart but also the vascular system, and this may increase serum NAG levels. Further studies are needed to address this possibility.
We conclude that elevated serum NAG activity is an effective indicator for the development of hypertension. Presumably, elevated serum NAG activity indicates an underlying or ongoing pathological or metabolic process in the cardiovascular system. Careful attention should be paid to subjects with elevated serum NAG activity even if it is not accompanied by hypertension or other detectable disease.
This study was supported by the Kimura Memorial Heart Foundation, Fukuoka, Japan. We are grateful to members of the Japan Medical Association of Ukiha, the elected officials and residents of Tanushimaru, and the team of physicians for their help in carrying out the health examinations.
- Received September 19, 1994.
- Revision received November 21, 1994.
- Accepted January 31, 1995.
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