Human small intestinal angiotensin-converting enzyme: intracellular transport, secretion and glycosylation

Human Angiotensin I-Converting Enzyme Produced by Different Cells: Classification of the SERS Spectra with Linear Discriminant Analysis

Angiotensin I-converting enzyme (ACE) is a peptidase widely presented in human tissues and biological fluids. ACE is a glycoprotein containing 17 potential N-glycosylation sites which can be glycosylated in different ways due to post-translational modification of the protein in different cells. For the first time, surface-enhanced Raman scattering (SERS) spectra of human ACE from lungs, mainly produced by endothelial cells, ACE from heart, produced by endothelial heart cells and miofibroblasts, and ACE from seminal fluid, produced by epithelial cells, have been compared with full assignment. The ability to separate ACEs’ SERS spectra was demonstrated using the linear discriminant analysis (LDA) method with high accuracy. The intervals in the spectra with maximum contributions of the spectral features were determined and their contribution to the spectrum of each separate ACE was evaluated. Near 25 spectral features forming three intervals were enough for successful separation of the spectra of different ACEs. However, more spectral information could be obtained from analysis of 50 spectral features. Band assignment showed that several features did not correlate with band assignments to amino acids or peptides, which indicated the carbohydrate contribution to the final spectra. Analysis of SERS spectra could be beneficial for the detection of tissue-specific ACEs.

The Importance of Glycosylation in COVID-19 Infection

Coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is currently one of the major health problems worldwide. SARS-CoV-2 survival and virulence are shown to be impacted by glycans, covalently attached to proteins in a process of glycosylation, making glycans an area of interest in SARS-CoV-2 biology and COVID-19 infection. The SARS-CoV-2 uses its highly glycosylated spike (S) glycoproteins to bind to the cell surface receptor angiotensin-converting enzyme 2 (ACE2) glycoprotein and facilitate host cell entry. Viral glycosylation has wide-ranging roles in viral pathobiology, including mediating protein folding and stability, immune evasion, host receptor attachment, and cell entry. Modification of SARS-CoV-2 envelope membrane with glycans is important in host immune recognition and interaction between S and ACE2 glycoproteins. On the other hand, immunoglobulin G, a key molecule in immune response, shows a distinct glycosylation profile in COVID-19 infection and with increased disease severity. Hence, further studies on the role of glycosylation in SARS-CoV-2 infectivity and COVID-19 infection are needed for its successful prevention and treatment. This chapter focuses on recent findings on the importance of glycosylation in COVID-19 infection.

"Bottom-Up" Strategy for the Identification of Novel Soybean Peptides with Angiotensin-Converting Enzyme Inhibitory Activity

IAVPTGVA (Soy1) and LPYP are two soybean peptides, which display a multifunctional behavior, showing in vitro hypocholesterolemic and hypoglycemic activities. A preliminary screening of their structures using BIOPEP suggested that they might be potential angiotensin-converting enzyme (ACE) inhibitors. Therefore, a bottom-up-aided approach was developed in order to clarify the in vitro hypotensive activity. Soy1 and LPYP dropped the intestinal and renal ACE enzyme activity with IC50 values equal to 14.7 ± 0.28 and 5.0 ± 0.28 μM (Caco-2 cells), and 6.0 ± 0.35 and 6.8 ± 0.20 μM (HK-2 cells), respectively. In parallel, a molecular modeling study suggested their capability to act as competitive inhibitors of this enzyme. Finally, in order to increase both their stability and hypotensive properties, a suitable strategy for the harmless control of their release from a nanomaterial was developed through their encapsulation into the RADA16-assembling peptide.

The Secretion and Action of Brush Border Enzymes in the Mammalian Small Intestine

Microvilli are conventionally regarded as an extension of the small intestinal absorptive surface, but they are also, as latterly discovered, a launching pad for brush border digestive enzymes. Recent work has demonstrated that motor elements of the microvillus cytoskeleton operate to displace the apical membrane toward the apex of the microvillus, where it vesiculates and is shed into the periapical space. Catalytically active brush border digestive enzymes remain incorporated within the membranes of these vesicles, which shifts the site of BB digestion from the surface of the enterocyte to the periapical space. This process enables nutrient hydrolysis to occur adjacent to the membrane in a pre-absorptive step. The characterization of BB digestive enzymes is influenced by the way in which these enzymes are anchored to the apical membranes of microvilli, their subsequent shedding in membrane vesicles, and their differing susceptibilities to cleavage from the component membranes. In addition, the presence of active intracellular components of these enzymes complicates their quantitative assay and the elucidation of their dynamics. This review summarizes the ontogeny and regulation of BB digestive enzymes and what is known of their kinetics and their action in the peripheral and axial regions of the small intestinal lumen.

Angiotensin II-generating enzymes, angiotensin-converting enzyme (ACE) and mast cell chymase (CMA1), in gastric inflammation may be regulated by H. pylori and associated cytokines

BACKGROUND

The local angiotensin II system (LAS) has numerous functions, including the regulation of growth and differentiation in the gastrointestinal tract. Angiotensin II (AngII) may be generated by angiotensin-I-converting enzyme (ACE) or mast cell chymase (CMA1) and plays an important role in inflammatory processes, although opinions differ as to which AngII-generating enzyme is primarily associated with AngII-mediated effects. ACE inhibitors have been shown to have a protective or healing effect on gastric ulcers and colitis in animal models, which could be related to the local expression of ACE.

METHODS

The localisation of ACE and CMA1 was examined immunohistochemically in Helicobacter pylori gastritis, non-H. pylori gastritis, gastric ulcers and non-lesional gastric tissues. Using real-time qRT-PCR, ACE- and CMA1-mRNA expression in gastric cell lines were examined and changes in ACE levels after exposure to H. pylori or cytokines (IL-1beta, IL-6, IL-8, TNF, TGFbeta1) were quantified.

RESULTS

ACE and CMA1 were not expressed in the non-lesional foveolar epithelium. Cytoplasmic staining for ACE in fundic chief cells, and apical membranous expression of ACE in the mucin-secreting cells of the antral and pyloric region was observed. ACE was found in endothelial cells of the gastric ulcer granulation tissue and CMA1 was strongly expressed in mast cells. ACE but not CMA1 was expressed in the MKN28, N87 and MKN45 gastric cell lines, and ACE mRNA expression was regulated by both H. pylori and the cytokines.

CONCLUSIONS

ACE in the gastric mucosa and the microvasculature of granulation tissue may represent a novel therapeutic target for the promotion of healing processes in gastritis and ulceration using ACE inhibitors or AT1R antagonists.

A genome-wide association study identifies new loci for ACE activity: potential implications for response to ACE inhibitor

Because angiotensin-converting enzyme (ACE) activity is implicated widely in biological systems, we aimed to identify its novel quantitative trait loci for the purposes of understanding ACE activity regulation and pharmacogenetics relating to ACE inhibitor (ACEI). We performed a two-stage genome-wide association study: (1) from 400 young-onset hypertension (YOH) subjects and (2) a confirmation study with an additional 623 YOH subjects. In the first stage, eight single nucleotide polymorphisms (SNPs) of the ACE structural gene and one SNP of ABO genes were significantly associated with ACE activity. SNP rs4343 in exon17 near the well-known insertion/deletion polymorphism had the strongest association. We confirmed in the second stage that three SNPs: rs4343 in ACE gene (P=3.0 x 10⁻²⁵), rs495828 (P=3.5 x 10⁻⁸) and rs8176746 (P=9.3 x 10⁻⁵) in ABO gene were significantly associated with ACE activity. We further replicated the association between ABO genotype/blood types and ACE activity in an independent YOH family study (428 hypertension pedigrees), and showed a potential differential blood pressure response to ACEI in subjects with varied numbers of ACE-activity-raising alleles. These findings may broaden our understanding of the mechanisms controlling ACE activity and advance our pharmacogenetic knowledge on ACEI.

Intestinal intraepithelial lymphocyte derived angiotensin converting enzyme modulates epithelial cell apoptosis

BACKGROUND & AIMS

Intestinal adaptation in short bowel syndrome (SBS) consists of increased epithelial cell (EC) proliferation as well as apoptosis. Previous microarray analyses of intraepithelial lymphocytes (IEL) gene expression after SBS showed an increased expression of angiotensin converting enzyme (ACE). Because ACE has been shown to promote alveolar EC apoptosis, we examined if IEL-derived ACE plays a role in intestinal EC apoptosis.

METHODS

Mice underwent either a 70% mid-intestinal resection (SBS group) or a transection (Sham group) and were studied at 7 days. ACE expression was measured, and ACE inhibition (ACE-I, enalaprilat) was used to assess ACE function.

RESULTS

IEL-derived ACE was significantly elevated in SBS mice. The addition of an ACE-I to SBS mice resulted in a significant decline in EC apoptosis. To address a possible mechanism, tumor necrosis factor alpha (TNF-alpha) mRNA expression was measured. TNF-alpha was significantly increased in SBS mice, and decreased with ACE-I. Interestingly, ACE-I was not able to decrease EC apoptosis in TNF-alpha knockout mice.

CONCLUSIONS

This study shows a previously undescribed expression of ACE by IEL. SBS was associated with an increase in IEL-derived ACE. ACE appears to be associated with an up-regulation of intestinal EC apoptosis. ACE-I significantly decreased EC apoptosis.

Structural determinants required for apical sorting of an intestinal brush-border membrane protein

The distinct protein and lipid constituents of the apical and basolateral membranes in polarized cells are sorted by specific signals. O-Glycosylation of a highly polarized intestinal brush-border protein sucrase isomaltase is implicated in its apical sorting through interaction with sphingolipid-cholesterol microdomains. We characterized the structural determinants required for this mechanism by focusing on two major domains in pro-SI, the membrane anchor and the Ser/Thr-rich stalk domain. Deletion mutants lacking either domain, pro-SI(DeltaST) (stalk-free) and pro-SI(DeltaMA) (membrane anchor-free), were constructed and expressed in polarized Madin-Darby canine kidney cells. In the absence of the membrane anchoring domain, pro-SI(DeltaMA) does not associate with lipid rafts and the mutant is randomly delivered to both membranes. Therefore, the O-glycosylated stalk region is not sufficient per se for the high fidelity of apical sorting of pro-SI. Pro-SI(DeltaST) does not associate either with lipid rafts and its targeting behavior is similar to that of pro-SI(DeltaMA). Only wild type pro-SI containing both determinants, the stalk region and membrane anchor, associates with lipid microdomains and is targeted correctly to the apical membrane. However, not all sequences in the stalk region are required for apical sorting. Only O-glycosylation of a stretch of 12 amino acids (Ala(37)-Pro(48)) juxtapose the membrane anchor is required in conjunction with the membrane anchoring domain for correct targeting of pro-SI to the apical membrane. Other O-glycosylated domains within the stalk (Ala(49)-Pro(57)) are not sufficient for apical sorting. We conclude that the recognition signal for apical sorting of pro-SI comprises O-glycosylation of the Ala(37)-Pro(48) stretch and requires the presence of the membrane anchoring domain.

Acetic acid suppresses the increase in disaccharidase activity that occurs during culture of caco-2 cells

To understand how blood glucose level is lowered by oral administration of vinegar, we examined effects of acetic acid on glucose transport and disaccharidase activity in Caco-2 cells. Cells were cultured for 15 d in a medium containing 5 mmol/L of acetic acid. This chronic treatment did not affect cell growth or viability, and furthermore, apoptotic cell death was not observed. Glucose transport, evaluated with a nonmetabolizable substrate, 3-O-methyl glucose, also was not affected. However, the increase of sucrase activity observed in control cells (no acetic acid) was significantly suppressed by acetic acid (P < 0.01). Acetic acid suppressed sucrase activity in concentration- and time-dependent manners. Similar treatments (5 mmol/L and 15 d) with other organic acids such as citric, succinic, L-maric, L-lactic, L-tartaric and itaconic acids, did not suppress the increase in sucrase activity. Acetic acid treatment (5 mmol/L and 15 d) significantly decreased the activities of disaccharidases (sucrase, maltase, trehalase and lactase) and angiotensin-I-converting enzyme, whereas the activities of other hydrolases (alkaline phosphatase, aminopeptidase-N, dipeptidylpeptidase-IV and gamma-glutamyltranspeptidase) were not affected. To understand mechanisms underlying the suppression of disaccharidase activity by acetic acid, Northern and Western analyses of the sucrase-isomaltase complex were performed. Acetic acid did not affect the de novo synthesis of this complex at either the transcriptional or translational levels. The antihyperglycemic effect of acetic acid may be partially due to the suppression of disaccharidase activity. This suppression seems to occur during the post-translational processing.

Temporal association of the N- and O-linked glycosylation events and their implication in the polarized sorting of intestinal brush border sucrase-isomaltase, aminopeptidase N, and dipeptidyl peptidase IV

The temporal association between O-glycosylation and processing of N-linked glycans in the Golgi apparatus as well as the implication of these events in the polarized sorting of three brush border proteins has been the subject of the current investigation. O-Glycosylation of pro-sucrase-isomaltase (pro-SI), aminopeptidase N (ApN), and dipeptidyl peptidase IV (DPPIV) is drastically reduced when processing of the mannose-rich N-linked glycans is blocked by deoxymannojirimycin, an inhibitor of the Golgi-located mannosidase I. By contrast, O-glycosylation is not affected in the presence of swainsonine, an inhibitor of Golgi mannosidase II. The results indicate that removal of the outermost mannose residues by mannosidase I from the mannose-rich N-linked glycans is required before O-glycosylation can ensue. On the other hand, subsequent mannose residues in the core chain impose no sterical constraints on the progression of O-glycosylation. Reduction or modification of N- and O-glycosylation do not affect the transport of pro-SI, ApN, or DPPIV to the cell surface per se. However, the polarized sorting of two of these proteins, pro-SI and DPPIV, to the apical membrane is substantially altered when O-glycans are not completely processed, while the sorting of ApN is not affected. The processing of N-linked glycans, on the other hand, has no influence on sorting of all three proteins. The results indicate that O-linked carbohydrates are at least a part of the sorting mechanism of pro-SI and DPPIV. The sorting of ApN implicates neither O-linked nor N-linked glycans and is driven most likely by carbohydrate-independent mechanisms.

The role of the protein glycosylation state in the control of cellular transport of the amyloid beta precursor protein

The amyloid beta precursor protein can exist as both a membrane-bound and a secreted protein, with the former having the potential to generate the amyloid beta peptide present in the neuritic plaques which are characteristic of Alzheimer's disease. In this study, we have used a clone of the AtT20 mouse pituitary cell line which expresses high levels of the amyloid beta precursor protein to characterize the glycosylation state of the secreted and membrane-bound forms of the protein and to examine the role of post-translational modifications in protein processing. Lectin blot analysis of immunoprecipitated amyloid beta precursor protein demonstrated that the soluble form of the protein contains significant amounts of sialic acid, with the lectin staining being reduced in the particulate cellular fractions. Treatment of the cells with mannosidase inhibitors to interfere with the formation of complex-type N-linked glycans resulted in a decrease in secreted amyloid beta precursor protein and an increase in the level of the cellular form of the protein. The increase in amyloid beta precursor protein levels in the cellular fraction was accompanied by an increase in perinuclear staining. Furthermore, cells overexpressing the alpha2,6(N)-sialyltransferase enzyme also demonstrated an increase in amyloid beta precursor protein secretion. These results suggest that the presence of terminal sialic acid residues on complex-type N-glycans may be required for the optimal transport of the amyloid beta precursor protein from the Golgi to the cell membrane with the subsequent cleavage to generate the secreted form of the protein.

Identification of N-linked glycosylation sites in human testis angiotensin-converting enzyme and expression of an active deglycosylated form

The sites of glycosylation of Chinese hamster ovary cell expressed testicular angiotensin-converting enzyme (tACE) have been determined by matrix-assisted laser desorption ionization/time of flight/mass spectrometry of peptides generated by proteolytic and cyanogen bromide digestion. Two of the seven potential N-linked glycosylation sites, Asn90 and Asn109, were found to be fully glycosylated by analysis of peptides before and after treatment with a series of glycosidases and with endoproteinase Asp-N. The mass spectra of the glycopeptides exhibit characteristic clusters of peaks which indicate the N-linked glycans in tACE to be mostly of the biantennary, fucosylated complex type. This structural information was used to demonstrate that three other sites, Asn155, Asn337, and Asn586, are partially glycosylated, whereas Asn72 appears to be fully glycosylated. The only potential site that was not modified is Asn620. Sequence analysis of tryptic peptides obtained from somatic ACE (human kidney) identified six glycosylated and one unglycosylated Asn. Only one of these glycosylation sites had a counterpart in tACE. Comparison of the two proteins reveals a pattern in which amino-terminal N-linked sites are preferred. The functional significance of glycosylation was examined with a tACE mutant lacking the O-glycan-rich first amino-terminal 36 residues and truncated at Ser625. When expressed in the presence of the alpha-glucosidase I inhibitor N-butyldeoxynojirimycin and treated with endoglycosidase H to remove all but the terminal N-acetylglucosamine residues, it retained full enzymatic activity, was electrophoretically homogeneous, and is a good candidate for crystallographic studies.

Affinity of angiotensin I-converting enzyme (ACE) inhibitors for N- and C-binding sites of human ACE is different in heart, lung, arteries, and veins

Angiotensin-converting enzyme (ACE) has two enzymatically active domains: a C-domain in the carboxy terminal region and an N-domain in the amino terminal region. We based the pharmacologic characterization of these sites on the rat testis-lung model. In testis, only a truncate form of ACE is present (C-site), whereas both N- and C-sites are present in lung. In this model, captopril was shown to be N-selective and delaprilat to be C-selective. Ro 31-8472, a cilazapril derivative, and enalaprilat proved to be not site selective. We used these drugs to evaluate the affinity of C and N sites in various human tissues involved in the cardiovascular actions of ACE and used [125I]Ro31-8472 as ligand. The number and affinity of ACE binding sites were 17,680 +/- 2,345 fmol/mg protein (Kd = 0.32 +/- 0.04 nM) in lung, 560 +/- 65 (Kd = 0.36 +/- 0.05 nM) in heart, 237 +/- 51 (Kd = 0.37 +/- 0.06 nM) in coronary artery, 236 +/- 63 (Kd = 0.14 +/- 0.05 nM) in saphenous vein, and 603 +/- 121 (Kd = 0.50 +/- 0.06 nM) in mammary artery. The affinity (pKi) of captopril for the N sites ranged from 9.40 +/- 0.14 (lung) to 8.41 +/- 0.10 (coronary artery). The affinity for the C-site by delaprilat ranged from 9.97 +/- 0.15 (coronary artery) to 9.10 +/- 0.14 (mammary artery). Therefore, the affinity of C- and N-sites of ACE for ACE inhibitor (ACEI) drugs is different according to the organ involved. Because ACE is a glycosylated enzyme and glycosylation is organ dependent, we suggest that organ-specific glycosylation affects the binding characteristics of ACE inhibitors to N- or C-site of human tissular ACE.

Guinea pig intestinal phospholipase B: protein expression during enterocyte maturation and effects of N-oligosaccharide removal on enzymatic activities and protein stability

Guinea pig phospholipase B (PLB) is an intestinal brush-border hydrolase displaying a broad substrate specificity towards various dietary lipids. PLB was detected by immunoblotting as a single 140-kDa polypeptide in all cell populations isolated from guinea pig intestinal mucosa, but increased in parallel to its activity from undifferentiated to mature cells, the specific activity of the enzyme remaining constant. Moreover, N-glycosylation, which contributed to 23% of the apparent molecular mass, was identical along the cell differentiation axis. In all cell fractions, N-linked sugar chains were of the complex type, since they were removed by N-glycosidase F, whereas PLB remained insensitive to endoglycosidase H. Moreover, lack of O-glycosylation was demonstrated by the insensitivity of PLB to O-glycosidase and by its failure to interact with Helix pomatia lectin after prior treatment with neuraminidase or alpha-fucosidase. Enzymatic removal of sugar chains reduced phospholipase A2, lysophospholipase and diacylglycerol lipase activities by 27-35%, kinetic analysis indicating a decrease in apparent Vmax values for the three enzymatic activities, whereas the Km remained unchanged. Finally, the carbohydrate-depleted form of PLB did not display gross changes in thermal stability, in contrast to PLB from microorganisms previously investigated. Our data indicate that the high level of PLB N-glycosylation is poorly related to its biological function. Whether carbohydrate chains are involved in proper targeting of the enzyme to the brush-border membrane remains to be established.

Secretion of human intestinal angiotensin-converting enzyme and its association with the differentiation state of intestinal cells

Human angiotensin I-converting enzyme (ACE) exists in intestinal epithelial cells as a membrane-bound (ACEm) and secretory glycoprotein (ACEsec). The electrophoretic mobilities of ACEsec and ACEm on SDS/polyacrylamide gels are similar; the N-deglycosylated ACEsec and ACEm, in contrast, display slight differences in their apparent molecular masses, indicating that the carbohydrate contents of ACEsec and ACEm are different. Moreover, ACEsec is solely N-glycosylated whereas ACEm is N- and O-glycosylated, assessed by lectin binding studies. Spontaneous release of ACEsec is achieved by incubation of brush border membranes at 37 degrees C. Aprotinin, leupeptin and EDTA partly inhibit the generation of ACEsec, strongly suggesting that ACEsec is generated from ACEm by proteolytic cleavage. The expression levels of ACEsec in the intestine may be associated with the differentiation state of mucosal cells. Thus ACEsec is more abundant than ACEm in immature non-epithelial crypt cells of patients with coeliac disease. Well-differentiated epithelial cells, by contrast, contain predominantly ACEm. The variations in the proportions of cleaved ACEsec in differentiated and non-differentiated cells may be due to varying levels of the cleaving protease. Alternatively, because epithelial cell differentiation is accompanied by alterations in the levels of oligosaccharyltransferases, the results suggest a critical role for the glycosylation pattern of ACEm in its susceptibility to the putative cleaving protease.

Congenital sucrase-isomaltase deficiency. Identification of a glutamine to proline substitution that leads to a transport block of sucrase-isomaltase in a pre-Golgi compartment

Congenital sucrase-isomaltase deficiency is an example of a disease in which mutant phenotypes generate transport-incompetent molecules. Here, we analyze at the molecular level a phenotype of congenital sucrase-isomaltase deficiency in which sucrase-isomaltase (SI) is not transported to the brush border membrane but accumulates as a mannose-rich precursor in the endoplasmic reticulum (ER), ER-Golgi intermediate compartment, and the cis-Golgi, where it is finally degraded. A 6-kb clone containing the full-length cDNA encoding SI was isolated from the patient's intestinal tissue and from normal controls. Sequencing of the cDNA revealed a single mutation, A/C at nucleotide 3298 in the coding region of the sucrase subunit of the enzyme complex. The mutation leads to a substitution of the glutamine residue by a proline at amino acid 1098 (Q1098P). The Q1098P mutation lies in a region that is highly conserved between sucrase and isomaltase from different species and several other structurally and functionally related proteins. This is the first report that characterizes a point mutation in the SI gene that is responsible for the transport incompetence of SI and for its retention between the ER and the Golgi.

Carboxydipeptidase from Boophilus microplus: a "concealed" antigen with similarity to angiotensin-converting enzyme

A protein, Bm91, which was first identified as a protective vaccine antigen from the tick Boophilus microplus, has regions of very strong amino acid sequence similarity to mammalian carboxydipeptidases or angiotensin converting enzymes (ACE; E.C. 3.4.15.1). This protein is now shown to share many biochemical and enzymatic properties with mammalian carboxydipeptidases. It is enzymatically active in a conventional assay for ACE using hippuryl-Gly-Gly as substrate. The hydrolysis of the C-terminal nonapeptide of the insulin B chain proceeds by sequential removal of carboxy-terminal dipeptides. The similarities extend to the dependence of activity on pH and added salt. Bm91 is inhibited by two well-characterized inhibitors of the mammalian enzymes, the drug Captopril and a nonapeptide, and the inhibition occurs in similar concentration ranges to those effective with the mammalian enzymes. However, the natural substrates of the tick enzyme are unknown. Angiotensin I itself is a poor substrate and the enzyme's natural substrates are likely to be one or more of the pharmacologically active peptides occurring in insects and arthropods.

Angiotensin I converting enzyme activity in uranyl nitrate induced acute renal failure in rats

Angiotensin I converting enzyme (ACE) was measured in urine, serum, and tissues from rats with acute renal failure (ARF) induced by a single subcutaneous injection (15 mg/kg BW) of uranyl nitrate (UN). Urine was collected daily until day 5, when rats were sacrificed by decapitation for the obtention of blood serum and tissues. Other groups of rats were sacrificed on days 1 and 2. These rats showed proteinuria and polyuria. The damage to the kidney proximal tubule was shown by (a) histological analysis at light and electron microscopy levels on days 1, 2, and 5, (b) the increase in urinary excretion of dipeptidyl aminopeptidase IV and N-acetyl-beta-D-glucosaminidase on days 1-5, and (c) the low molecular weight proteinuria pattern on day 1. In addition, the histological analysis at the ultrastructural level showed normal glomeruli appearance on days 1 and 2, but structural alterations on day 5. These data suggest that the increased urinary excretion of enzymes and proteins is a consequence of the tubular injury on days 1 and 2, and of tubular and glomerular injury on day 5. ACE activity increased in urine on days 1-5 and in serum on day 5. Tissue ACE activity increased in lung, small intestine, and adrenal glands; and remained unchanged in testis, aorta, brain, kidney, heart, and liver. Our data suggest that: (a) the increase in serum ACE may be secondary to the changes in tissue ACE activity, and (b) the urine ACE increase may be due to the kidney proximal tubule damage. This work supports the contention that an increase in urine ACE may be an indicator of injury to the proximal tubule.