Zonal distribution of fatty acid synthase in liver parenchyma of male and female rats

Abstract

1. A sensitive radiochemical assay was established to determine the activity of fatty acid synthase in microdissected liver tissue of less than 1 pg dry mass.2. In female rats, the enzyme activity in perivenous tissue was twice that in periportal liver tissue while it was homogeneously distributed in livers of male animals. The overall activity was higher in female than in male animals.3. The absolute activity, as well as the perivenous/periportal ratio, was reduced during starvation and in diabetes. They were greatly increased after refeeding to values above those observed in animals during normal feeding.4. Ovarectomy or administration of testosterone to female rats resulted in a significant reduction of the zonal heterogeneity.5. Castration or administration of estradiol to male animals was followed by an increase in the enzyme activity exclusively in the perivenous tissue, resulting in a zonal heterogeneity as observed in female rats.Periportal and perivenous hepatocytes exhibit different metabolic capacities : enzyme activities involved in gluconeogenesis, urea synthesis from amino acids and bile formation are higher in the periportal zone of the rat liver acinus; enzyme activities involved in glycolysis and biotransformation are higher in the perivenous zone [I -41. Therefore, it was postulated that periportal hepatocytes catalyze net glucose formation from lactate and alanine and perivenous hepatocytes catalyze net glucose degradation.This hypothesis was confirmed by calculation of metabolic flux rates in the different zones of the acinus [ S ] as well as by measurement of the zonal metabolic activities in isolated perfused livers using miniature oxygen electrodes [6, 71 or histochemical techniques [8, 91. Glycolysis and oxidative decarboxylation of pyruvate supply acetyl-CoA, which serves as a substrate for the synthesis of fatty acids during carbohydrate feeding of rats. Thus, the localization of the liponeogenic key enzymes, ATP citrate lyase [lo] and acetyl-CoA carboxylase [ll], predominantly in the perivenous zone supports the concept of the metabolic zonation in liver. A similar zonal distribution was described in female rats for glucose-6-phosphate dehydrogenase [12, 131 and for the 'malic' enzyme [13, 141, which supply NADPH for fatty acid synthesis and hydroxylation reactions.The zonal distribution of fatty acid synthase, the third classical enzyme of liponeogenesis, is demonstrated in the Correspondence to N. Katz,

Hepatocyte Heterogeneity in the Metabolism of Fatty Acids: Discrepancies on Zonation of Acetyl-CoA Carboxylase

“…Previous studies with histochemical and microchemical techniques have shown a similar zonation of the ancil lary NADPH-generating enzymes although with some controversy, probably due to con tamination from nonparenchymal cells [5]. By the same method, zonation of FAS was found to be similar to that of ACC in the female, but not in the male rat [6], Applying…”

“…Most results obtained by the microanaly sis technique using microdissected samples are expressed per milligram of dry weight, as is the case in the studies by Katz et al [3,4,6], while the studies on isolated hepatocytes are usually expressed per milligram of total protein [9,10,16] or in some studies per milligram of DNA [7,16]. Contrary to this, the results of Evans et al [11,12] on the zon ation of ACC, applying digitonin-pulse per fusion [14], are expressed per milligram of cytosolic protein or as the true specific activ ity, since the assay was performed also with the isolated ACC protein.…”

“…Under standard conditions the microdissection/microanaly sis technique is also aimed at sampling some 30-50% of the microcirculatory zone [3,4,6]. Potentially, however, the microdissection technique can sample very accurately, as in deed demonstrated by several studies aimed at describing the shape of the enzyme activ ity gradients along the sinusoid [26,27], However, such studies have not yet been per-fer to an initial PP and PV subzone, consid erably smaller than the zones examined by the cell isolation technique and microdissec tion studies reviewed here.…”

Participation of Two Members of the Very Long-chain Acyl-CoA Synthetase Family in Bile Acid Synthesis and Recycling

“…Hepatic lipogenic enzymes appear to have a more perivenous distribution (32,33), whereas the rate-limiting step in bile acid synthesis, cholesterol 7␣-hydroxylase, is found mainly in periportal hepatocytes (34). Expression of hBACS was also found to have a more periportal distribution.…”

Hepatocyte Heterogeneity in the Metabolism of Carbohydrates

“…The techniques used in the study of hepa tocellular heterogeneity have been reviewed previously [10,23], The distribution of en-zyme activities and amounts was studied us ing histochemical [1][2][3][4], immunohistochem ical [24][25][26] and microdissective/microbiochemical techniques [26][27][28][29][30][31][32][33][34][35][36][37] as well as the digitonin dual pulse perfusion method [38], Periportal and perivenous hepatocytes were characterized functionally by using ortho-(= antera-) and retrograde liver perfusions, periportal-and perivenous-enriched hepato cyte populations [39][40][41][42][43] and periportal-like and perivenous-like hepatocytes induced in long-term primary culture [44], They were also characterized by applying noninvasive techniques with micro-light guides and with miniature oxygen electrodes positioned in periportal and perivenous areas at the liver surface for the measurement of surface fluo rescence and surface reflectance and of sur face oxygen tension, respectively [8] , 20 40 60 80 100 100 80 60 40 20 Blood level (%) Fig. 1.…”