Characterization of murine liver-derived inhibitory protein

Development of a sandwich ELISA test for arginase measurement based on monoclonal antibodies

Human arginase was purified from liver and two monoclonal antibodies (MAbs), HA1 and HA2, were produced by fusion of spleen cells from an arginase-immunized BALB/c mouse and the NS-1 myeloma cell line. Both MAbs were of the IgG3 subclass and contained the kappa light chain. HA1 inhibited arginase activity, suggesting that it binds to the arginase catalytic site. HA1 and a horseradish peroxidase-conjugated polyclonal rabbit anti-human arginase antibody were used to develop a sandwich enzyme-linked immunoadsorbent assay (ELISA) for the quantification of human arginase, which can be used in the 1 to 300 ng/mL range. Because of its sensitivity and specificity, this MAb can be successfully applied to the ELISA quantification of arginase in serum and culture supernatants.

Comparative properties of arginases

Arginase is a primordial enzyme, widely distributed in the biosphere and represented in all primary kingdoms. It plays a critical role in the hepatic metabolism of most higher organisms as a cardinal component of the urea cycle. Additionally, it occurs in numerous organisms and tissues where there is no functioning urea cycle. Many extrahepatic tissues have been shown to contain a second form of arginase, closely related to the hepatic enzyme but encoded by a distinct gene or genes and involved in a host of physiological roles. A variety of functions has been proposed for the "extrahepatic" arginases over the last three decades. In recent years, interest in arginase has been stimulated by a demonstrated involvement in the metabolism of the ubiquitous and multifaceted molecule nitric oxide. Molecular biology has begun to furnish new clues to the disparate functions of arginases in different environments and organisms. Comparative studies of arginase sequences are also beginning to elucidate the comparative evolution of arginases, their molecular structures and the nature of their catalytic mechanism. Further studies have sought to clarify the involvement of arginase in human disease. This review presents an outline of the current state of arginase research by giving a comparative overview of arginases and their associated properties.

Immunosuppressive factor from liver induces apoptosis in thymoma EL-4 cells but not normal MHC class II-specific T lymphocytes

An endogenously produced immunosuppressive factor (ISFnp, immunosuppressive factor-neutral protein), inducing a decrease in viability of thymoma EL-4 cells in vitro, was isolated from murine liver using ion exchange, gel filtration and hydrogen-bonding chromatography. Polyclonal rabbit antibodies against this factor were developed and attached to periodate-activated Sepharose CL-6B. The immunoaffine sorbent obtained significantly depleted the biological activity of ISFnp from tested fractions. The factor shows liver-specific location, an M(r) of about 70-80 kDa and consists of 2 subunits (40 and 42 kDa) as determined by SDS-PAGE and Western blotting. ISFnp induced DNA degradation in EL-4 cells similar to the cleavage of DNA onto olygonucleosomal fragments in dexamethasone-treated thymocytes. This DNA degradation preceded lysis of thymoma cells, suggesting an induction of apoptosis in ISFnp-treated EL-4 cells. Addition of the factor into primary mixed lymphocyte culture (MLC) strongly inhibited proliferative response but failed to induce any decrease in the ability of normal MHC class II-specific alloreactive cells to respond in the secondary MLC. Moreover, addition of ISFnp into primary MLC on the peak of proliferative response resulted in augmentation of secondary responses of primed cells as compared with the same quantities of primed cells from untreated cultures. These results suggest a possible role of liver both in deletion of transformed clones of T lymphocytes and formation of allospecific memory T cells.

Plasma arginase concentration measured by an enzyme-linked immunosorbent assay (ELISA) in normal adult population

Human liver arginase has many biological effects on lymphocytes, macrophages, liver cells, and tumor cells, in addition to its major role in the liver urea cycle. We have developed a sandwich enzyme-linked immunosorbent assay (ELISA) method to quantitate arginase concentrations in plasma that can be applied to various body fluids. The sensitivity was 2.5 ng/mL. The coefficients of variation were good both in intra- and inter-assay. Using this method to study the stability of an arginase preparation, we found that plasma arginase was stable for only 1 or 2 days even at temperatures as low as 4 degrees C. The mean plasma level was 41.0 +/- 3.3 ng/mL (mean +/- SE) in 143 normal subjects. There was no age difference in the general population and in the male group. However, in the female group, the plasma arginase level increased with age (p = 0.05). Its biological significance was unclear. As a whole, the ELISA method for the measurement of arginase concentration in the body fluid is convenient and reliable.

Clinical significance of arginase in colorectal cancer

Arginase, a potent immune inhibitor, existed in much greater abundance in the cytoplasm of cancer cells than in normal cells. Serum arginase levels from 31 patients with colorectal adenocarcinoma were determined by using enzyme immunoassay (mean +/- standard error = 18.96 +/- 4.83 ng/ml) and showed to be significantly higher than levels from control subjects (n = 115, 3.09 +/- 0.22 ng/ml) (P less than 0.005). Surgical samples of 15 patients were individually homogenized and assayed by the same method and revealed that the arginase level in tissues with colorectal cancer was two times greater than the level found in normal mucosal tissues (1.74 +/- 0.31 micrograms/g tissue versus 0.77 +/- 0.09 micrograms/g tissue, P less than 0.005). However, the serum arginase levels in patients with colorectal cancer were independent of their carcinoembryonic antigen (CEA) levels (n = 27, arginase 11.81 +/- 1.88 ng/ml, CEA 17.31 +/- 4.24 ng/ml, r = 0.084, P = 0.666). The results suggested that serum arginase level can be a valuable criterion for preoperative evaluation and possibly postoperative follow-up study. It can also combine with CEA determination to intensify the clinical assessment for colorectal cancer.

Inhibition of lymphocyte proliferation by liver arginase

The activities of thymidine kinase and uridine kinase (enzymes for pyrimidine salvage pathway) in phytohemagglutinin (PHA)--prestimulated lymphocytes were inhibited by arginase in a similar pattern to the inhibition on thymidine incorporation. Further study revealed that arginase did not directly affect the activities of these enzymes in the cell-free system. Thymidine kinase and uridine kinase activities of PHA-prestimulated lymphocytes were inhibited by arginase making their activities as low as that cultured in arginine-free RPMI-1640 medium. These results suggest that arginine-depletion in the culture medium is the primary mode of action of arginase on the inhibition of mitogen-stimulated lymphocyte proliferation.

Isolation and characterization of a novel liver-derived immunoinhibitory factor

Cytosolic extracts prepared from perfused whole liver or purified hepatocytes of C57BL/6 mice inhibited interleukin-2--and concanavalin A--induced spleen cell proliferation in vitro. In contrast, cytosolic extracts from purified nonparenchymal liver cells had no effect. Arginase and very-low-density lipoprotein were previously identified as two immuninhibitory substances present in liver cytosolic extracts. We demonstrated, however, that inhibitory activity remained after removal of very-low-density lipoprotein and arginase from liver cytosolic extract by repeated ultracentrifugation and gel filtration chromatography, respectively, suggesting the presence of another inhibitor. Further purification by anion-exchange chromatography and chromatofocusing led to the isolation of a novel liver-derived immunohibitory factor. This liver-derived immunoinhibitory factor is sensitive to pronase digestion and heat and acid treatment; it has an estimated isoelectric point of 8.25. The Mr of liver-derived immunoinhibitory factor is 28 kD as estimated from its migration on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, which is identical under both reducing and nonreducing conditions, indicating a monomeric nature of this protein. Amino acid composition analysis discloses that liver-derived immunoinhibitory factor is relatively rich in glycine and proline residues. Interleukin-2--induced spleen cell proliferation in vitro is inhibited by ths liver-derived immunoinhibitory factor, with a 50% inhibitory dose of 1.4 nmol/L. Furthermore, the biological activity of the liver-derived immunoinhibitory factor is not confined to mouse spleen cells, since the growth of B16 mouse melanoma and H35 rat hepatoma cells is also inhibited. A comparison with other liver-derived immunoinhibitors reported previously supports our claim that the liver-derived immunoinhibitory factor is a novel inhibitory protein.