Changes of platelet cell volumes in hypotonic solution

Tetrahydrobiopterin regulates monoamine neurotransmitter sulfonation

Monoamine neurotransmitters are among the hundreds of signaling small molecules whose target interactions are switched "on" and "off" via transfer of the sulfuryl-moiety (-SO3) from PAPS (3'-phosphoadenosine 5'-phosphosulfate) to the hydroxyls and amines of their scaffolds. These transfer reactions are catalyzed by a small family of broad-specificity enzymes-the human cytosolic sulfotransferases (SULTs). The first structure of a SULT allosteric-binding site (that of SULT1A1) has recently come to light. The site is conserved among SULT1 family members and is promiscuous-it binds catechins, a naturally occurring family of flavanols. Here, the catechin-binding site of SULT1A3, which sulfonates monoamine neurotransmitters, is modeled on that of 1A1 and used to screen in silico for endogenous metabolite 1A3 allosteres. Screening predicted a single high-affinity allostere, tetrahydrobiopterin (THB), an essential cofactor in monoamine neurotransmitter biosynthesis. THB is shown to bind and inhibit SULT1A3 with high affinity, 23 (±2) nM, and to bind weakly, if at all, to the four other major SULTs found in brain and liver. The structure of the THB-bound binding site is determined and confirms that THB binds the catechin site. A structural comparison of SULT1A3 with SULT1A1 (its immediate evolutionary progenitor) reveals how SULT1A3 acquired high affinity for THB and that the majority of residue changes needed to transform 1A1 into 1A3 are clustered at the allosteric and active sites. Finally, sequence records reveal that the coevolution of these sites played an essential role in the evolution of simian neurotransmitter metabolism.

Effects of taxol and colchicine on platelet membrane properties

The effects of the assembly-disassembly of microtubule on the membrane lipid fluidity, the fragmental motion of sulfhydryl groups of membrane proteins, and the functions of shape change and hypotonic shock response (shrinkage ratio) in human platelets were studied. We have employed electron spin resonance (ESR) utilizing spin labels for bilayer lipids or membrane proteins and microtubule reactive reagents to change microtubule assembly. Both, taxol, a microtubule stabilizing agent, and colchicine, a microtubule disrupting agent, did not affect the platelet membrane lipid fluidity detected by 5- or 16-doxylstearate. On the other hand, the mobility of sulfhydryl groups detected by 4-maleimide-tempo increased by taxol treatment and decreased by colchicine. Moreover, the temperature sensitivity of platelets below 20 degrees C decreased by taxol treatment, but unchanged by colchicine treatment. This behavior was similar to the shape change ability or the shrinkage ratio of platelets pre-treated with taxol, in which microtubule disassembly was inhibited due to taxol binding to microtubule. Therefore, it is confirmed that the assembly-disassembly state of microtubule which may control the ability of platelets to change shape, influences the mobility of sulfhydryl groups in platelet membrane proteins and its temperature sensitivity.

Membrane potential of stored platelets and its effect on platelet functions

Relationships among intracellular Ca2+ level, membrane potential in resting state and platelet functions were examined. Membrane potential of platelets was increased (hyperpolarized) during storage and decreased (depolarized) by 37 degrees C incubation with plasma after storage. The degree of depolarization was greater in fresh plasma than in stored plasma. Changes of platelet functions were reverse with the membrane potential change. Aggregation induced by ADP or collagen was enhanced by depolarization with increasing extracellular [K+]. Shrinkage ratio (hypotonic shock response) of stored platelet which were hyperpolarizing, exhibited optimal value when the membrane potential was slightly depolarized. Intracellular [Ca2+] was not affected by the extracellular [K+]. These results suggest that platelet sensitivity is controlled by the membrane potential in resting state.

Specific non-saturable binding of insulin by human platelets

Studies on the nature and characterization of the specific binding of 125I-insulin to intact human platelets have been undertaken. Although under conditions of physiological buffer osmolality, a binding equilibrium was attained in 4-6 h at 4 degrees C, at higher temperatures (17 degrees C or 24 degrees C) equilibrium was reached only in the presence of very high buffer osmolality or 25 mM NaF. Under conditions of normal osmolality and in the absence of NaF, binding at 17 degrees C was not saturable. This phenomenon was specific for insulin or insulin-like hormones (the insulin-like growth factors) and did not occur with 125I-labelled growth hormone, ACTH or beta-endorphin. The non-saturable uptake of insulin appeared due to an energy-dependent specific sequestration or internalization of insulin by mechanisms probably involving the microtubule system. This study emphasizes the need to restrict this non-saturable uptake of insulin if one wishes to adequately study the platelet plasma membrane receptors for insulin. These data also indicate that there is a major interaction of insulin and insulin-like hormones with normal human platelets and support previous demonstrations of major insulin effects on platelet physiology in both normal and diabetic states.