Titanium dioxide hogs the spotlight

In their unending quest to im- prove day-to-day life, researchers have processed a common pigment into a form that could lead to new generations of self-sterilizing bath- room tiles and antifog mirrors.

Each year, the United States pro- duces some 1.5 million tons of titani- um dioxide, a powder that’s widely used as a sunscreen and a white pig- ment for paints. In the past few years, chemists have also found that coatings of titanium dioxide can kill bacteria, catalyze the breakdown of dirt and toxic pollutants, and prevent water from building into tiny, fog-forming beads (SN: 3/21/98, p. 186).

But there’s a catch. Materials con- taining titanium dioxide-whether in- stalled in air ducts, toilets, or operating rooms-behave in these beneficial ways only when exposed to ultraviolet 0 radiation. Since only about 5 per- cent of sunlight and typical indoor light falls into the UV range, researchers of- ten use UV lamps to activate these ma- terials, says John T. Yates Jr. of the Uni- versity of Pittsburgh.

Now, a team of Japanese researchers has discovered that adding nitrogen ions to the titanium dioxide makes coatings that can destroy organic mole- cules in the presence of visible light or UV wavelengths. Also, a nitrogen- spiked titanium dioxide film holds wa- ter so tightly, even in visible light, that the liquid flattens out. In other words, water on the surface doesn’t form the tiny droplets that would cloud a bath- room or car mirror.

“This should allow the main part of the solar spectrum and even poor illu- mination of interior lighting to be used” with these materials, says Ryoji Asahi of Toyota Central R&D Laboratories in Nagakute. His team reports the results in the July 13 SCIENCE.

Previously, other researchers had made related materials by combining titanium dioxide and metals. Howev- er, nitrogen-enriched titanium dioxide coatings have benefits over metal-en- riched ones, such as better stability and lower cost, the team says.

Nitrogenenriched coating could r e place titanium dioxide materials that have been under development for sev- eral years, says Yates. Researchers have experimented with such materi- als for treating sewage and coating self- cleaning cars and windows, as well as uses in bathrooms and hospitals, says Yates. He and his colleagues now plan to test whether a coating might break down nerve gas on military vehicles as they sit in the sun. 4 Corman

Vitamin A calibrates a heart clock, 24-7 Teenagers can make it seem as if hor-

mones control the cycles of the heart. Now, scientists have proved it.

Researchers at the University of Penn- sylvania in Philadelphia have shown that a molecular clock in the circulatory sys- tem oscillates in a 24-hour rhythm in step with cells in the brain. This heart cycle responds to a hormone derived from vitamin A.

Scientists have known for years about regions of the brain where cells cycle through a steady cadence of activity, called a circadian rhythm, that regulates sleepwake cycles. The concentrations of molecules, called clock proteins, that drive this process also wax and wane over a 24-hour period.

Only in the past 3 years have scien- tists learned that the concentrations of such proteins also oscillate in organs beyond the brain, such as liver and kid- neys. In the June 29 CELL, Garret A. FitzGerald, Peter McNamara, and their colleagues report that such clock pro- teins also cycle in the heart-and even in blood vessels. They observed these cycles in aortas removed from mice and in human blood vessel cells grown in culture.

The new data are in step with findings that blood pressure cycles over a 24-hour period, peaking in the early morning, which is when most heart attacks occur.

Since the discovery of peripheral clocks, scientists have wondered how the clocks all stay in sync. That’s where vita- min A comes into the picture.

The body converts vitamin A into the hormone retinoic acid, which journeys through the bloodstream and into cells. There, it docks onto proteins called retinoic acid receptors. These receptors, in turn, enter the nucleus and affect the activity of genes. To do this, the recep- tors must bind to other proteins.

In what he describes as a molecular fishing expedition, McNamara found that retinoic acid receptors can bind to two clock proteins-Clock and Mop$. That suggested that retinoic acid could regulate circadian rhythms. When the team applied retinoic acid to cells, the receptor and these clock proteins stuck together more tightly.

This molecular clasp affects genes that control circadian rhythms. And it resets the clock. When the researchers applied retinoic acid to blood vessel cells in lab dishes or injected the substance into the bloodstream of mice, they found that the heart clock shifted backward by several hours. These results jibe with existing da- ta that retinoic acid and other hormones cycle up and down through the day.

The scientists now plan to test whether retinoic acid in the circulatory system mediates the circadian rhythm originat- ing in the brain or if the hormone modu-

lates the peripheral rhythm directly in re- sponse to cues such as food intake.

“I would expect that in the wake of this paper, scientists will find a whole variety of hormonal signals,” says Steve A. Kay of the Scripps Research Institute in La Jolla, Calif. “This is really an important piece of work.” He notes that the findings may have relevance for developing new means of controlling the body’s response to drugs, which can vary drastically over 24 hours. -C Schubert

Sticky platelets boost blood clots

Platelets are tiny, cell-like components in the blood that aggregate to form clots. They’re indispensable for closing wounds, but after injury caused by atherosclerosis in an artery, their automatic response can clog the vessel.

Scientists now report that people with platelets that tend to stick together are more likely to suffer complications from a common heart procedure than are those with less sticky platelets. In another study, researchers reveal that a genetic trait induces some people to make sticky platelets. The studies, which appear in the July 10 CIRCULATION, could change how physicians treat people for heart disease.

In the first study, researchers tested how readily platelets aggregated in blood from 112 heart patients and compared the results with the patients’ symptoms after a surgical procedure. The scientists first measured platelet reactions to adenosine diphosphate, a compound that abets platelet clumping. Then, they divided the group into the half with the more sticky, or reactive, platelets and the half with less reactive ones.

All the study volunteers underwent an- gioplasty, in which a balloon-tipped catheter opens a blocked coronary artery.

Three months after the procedure, 15 of the patients with sticky platelets had experienced complications such as chest pain or a heart attack. Only 4 of the pa- tients with less-reactive platelets had such complications. During these first 3 months, 10 people with sticky platelets needed another angioplasty, compared with only 2 in the other group, says study coauthor Samer S. Kabbani, a cardiologist at the University of Vermont and Fletcher Allen Health Care in Burlington.

People undergoing angioplasty take as- pirin indefinitely and usually get other an- ticlotting drugs for a month or more. Us- ing the platelet-reactivity test, “we can basically predict which people are going to have complications” and adjust medica- tion accordingly, Kabbani says.

In the second study, researchers inves- tigated the effect of genetic variations in

22 SCIENCE NEWS, VOL. 160 JULY 14,2001