ZigZAGging through fat stores.

When reading how in the 1950s and 1960s Burgi and Schmid (1) at Harvard purified and isolated from human blood plasma a glycoprotein by zinc precipitation, which is now best known as Zn2-glycoprotein (ZAG), one can only wonder about the complete and detailed biochemical approach and pure marvel of biochemistry at that time. In the present-day world of ZAG, this seminal paper deserves a status similar, for instance, to that of the work of Lowry and colleagues on protein determination (reviewed in Ref. 2). What Burgi and Schmid did not know, nor could they anticipate, was the role played by “their” glycoprotein in endocrinology and metabolism. At that time, discussions still concerned questions such as whether a cell could produce only one hormone or maybe more. Hormones came from designated glands such as the pituitary gland, the endocrine pancreas, or adrenals. Tissues and organs, such as the skin, heart, liver, and kidney, were only later revealed as sources of endocrine signals. Now, almost 50 yr later, production of multiple signals by a single cell type is the rule rather than the exception, and the notion that tissues other than the classical endocrine glands, tissue such as liver and adipose, are endocrine tissues is established. Because many signals in our body serve functions beyond the classical endocrine function, viz. as intracrines, autocrines, and paracrines, we should be alert to the possibility of finding signals in body fluids other than blood plasma and not be prejudiced with regards to function because of exotic appearances of as yet poorlydefined signal substances. Such a notion would certainly apply to adipose tissue as an endocrine tissue and the complex web of adipo(cyto)kines produced by fat and liver cells involved in regulation of our energy stores and communication between these stores, other peripheral tissues of mesenchymal origin such as bone, or the immune system, and the central nervous system (3, 4). Indeed, ZAG is a rather enigmatic and multidisciplinary protein; although well studied, we still do not know ZAG’s function(s) with any great certainty (3). It is a single-chain, glycosylated polypeptide of approximately 40 kDa, produced by a variety of tissues and found in various body fluids and in rather high total concentrations. Liver and adipose tissue are main sites of production. ZAG is a major histocompatibility complex (MHC) class I look-alike, although the peptide binding groove in ZAG is less peptide-specific than in MHC molecules and was shown to bind polyethylene glycol in its groove (3), in line with a derived wider functionality. Apparently, ZAG serves a role as receptor and as adipocytokine signal as well, and its promiscuity toward ligands may greatly add to the complexity of its presumed signal functions. The typical blood plasma concentration in normal subjects is 60 mg/liter (5), which translates, with an apparent molecular radius of 40 kDa, to 1.5 M. If all the ZAG in plasma was freely available for binding to receptors, then this would be a receptor with an unusually low affinity for an endocrine signal (mostly nanomolar affinities or lower still); seminal plasma concentrations may even exceed those in plasma 6-fold (6), leading to further questioning of the endocrine nature of ZAG. But what do we know about free levels in body fluids, ligand-dependent ZAGcomplexes, or receptor types? Reportedly, ZAG (then, tumor-derived lipid-mobilizing factor that proved to be very similar to ZAG) increases cAMP levels in adipocytes through interference with a 3-adrenoceptor based on pharmacological studies with specific agonists and antagonists (7, 8) and, although 250 times more ZAG was required to obtain similar stimulation as with 1 nM isoprenaline, still a “high-affinity” receptor was indicated in kinetic studies (Kd: 78 nM). The search for and identification of a ZAG-specific G protein-coupled receptor